rpc/test: wait for mempool CheckTx callback (#4908)

Fixes race conditions causing the following test failures:

```
=== RUN   TestUnconfirmedTxs
    TestUnconfirmedTxs: rpc_test.go:342:
                Error Trace:    rpc_test.go:342
                Error:          Not equal:
                                expected: 1
                                actual  : 0
                Test:           TestUnconfirmedTxs
    TestUnconfirmedTxs: rpc_test.go:343:
                Error Trace:    rpc_test.go:343
                Error:          Not equal:
                                expected: 1
                                actual  : 0
                Test:           TestUnconfirmedTxs
    TestUnconfirmedTxs: rpc_test.go:345:
                Error Trace:    rpc_test.go:345
                Error:          Not equal:
                                expected: types.Txs{types.Tx{0x39, 0x44, 0x4d, 0x6c, 0x4b, 0x66, 0x46, 0x78, 0x3d, 0x45, 0x33, 0x33, 0x68, 0x47, 0x6e, 0x79, 0x58}}
                                actual  : types.Txs(nil)

                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -1,4 +1,2 @@
                                -(types.Txs) (len=1) {
                                - (types.Tx) (len=17) Tx{39444D6C4B6646783D45333368476E7958}
                                -}
                                +(types.Txs) <nil>

                Test:           TestUnconfirmedTxs
    TestUnconfirmedTxs: rpc_test.go:342:
                Error Trace:    rpc_test.go:342
                Error:          Not equal:
                                expected: 1
                                actual  : 0
                Test:           TestUnconfirmedTxs
    TestUnconfirmedTxs: rpc_test.go:343:
                Error Trace:    rpc_test.go:343
                Error:          Not equal:
                                expected: 1
                                actual  : 0
                Test:           TestUnconfirmedTxs
    TestUnconfirmedTxs: rpc_test.go:345:
                Error Trace:    rpc_test.go:345
                Error:          Not equal:
                                expected: types.Txs{types.Tx{0x39, 0x44, 0x4d, 0x6c, 0x4b, 0x66, 0x46, 0x78, 0x3d, 0x45, 0x33, 0x33, 0x68, 0x47, 0x6e, 0x79, 0x58}}
                                actual  : types.Txs{}

                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -1,3 +1,2 @@
                                -(types.Txs) (len=1) {
                                - (types.Tx) (len=17) Tx{39444D6C4B6646783D45333368476E7958}
                                +(types.Txs) {
                                 }
                Test:           TestUnconfirmedTxs
--- FAIL: TestUnconfirmedTxs (0.20s)
=== RUN   TestNumUnconfirmedTxs
    TestNumUnconfirmedTxs: rpc_test.go:364:
                Error Trace:    rpc_test.go:364
                Error:          Not equal:
                                expected: 1
                                actual  : 0
                Test:           TestNumUnconfirmedTxs
    TestNumUnconfirmedTxs: rpc_test.go:365:
                Error Trace:    rpc_test.go:365
                Error:          Not equal:
                                expected: 1
                                actual  : 0
                Test:           TestNumUnconfirmedTxs
    TestNumUnconfirmedTxs: rpc_test.go:364:
                Error Trace:    rpc_test.go:364
                Error:          Not equal:
                                expected: 1
                                actual  : 0
                Test:           TestNumUnconfirmedTxs
    TestNumUnconfirmedTxs: rpc_test.go:365:
                Error Trace:    rpc_test.go:365
                Error:          Not equal:
                                expected: 1
                                actual  : 0
                Test:           TestNumUnconfirmedTxs
--- FAIL: TestNumUnconfirmedTxs (0.09s)
```

rpc/client/rpc_test.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 	"sync"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -331,14 +332,22 @@ func TestBroadcastTxCommit(t *testing.T) {
 func TestUnconfirmedTxs(t *testing.T) {
 	_, _, tx := MakeTxKV()
 
+	ch := make(chan *abci.Response, 1)
 	mempool := node.Mempool()
-	_ = mempool.CheckTx(tx, nil, mempl.TxInfo{})
+	err := mempool.CheckTx(tx, func(resp *abci.Response) { ch <- resp }, mempl.TxInfo{})
+	require.NoError(t, err)
 
-	for i, c := range GetClients() {
-		mc, ok := c.(client.MempoolClient)
-		require.True(t, ok, "%d", i)
+	// wait for tx to arrive in mempoool.
+	select {
+	case <-ch:
+	case <-time.After(5 * time.Second):
+		t.Error("Timed out waiting for CheckTx callback")
+	}
+
+	for _, c := range GetClients() {
+		mc := c.(client.MempoolClient)
 		res, err := mc.UnconfirmedTxs(1)
-		require.Nil(t, err, "%d: %+v", i, err)
+		require.NoError(t, err)
 
 		assert.Equal(t, 1, res.Count)
 		assert.Equal(t, 1, res.Total)
@@ -352,10 +361,19 @@ func TestUnconfirmedTxs(t *testing.T) {
 func TestNumUnconfirmedTxs(t *testing.T) {
 	_, _, tx := MakeTxKV()
 
+	ch := make(chan *abci.Response, 1)
 	mempool := node.Mempool()
-	_ = mempool.CheckTx(tx, nil, mempl.TxInfo{})
-	mempoolSize := mempool.Size()
+	err := mempool.CheckTx(tx, func(resp *abci.Response) { ch <- resp }, mempl.TxInfo{})
+	require.NoError(t, err)
 
+	// wait for tx to arrive in mempoool.
+	select {
+	case <-ch:
+	case <-time.After(5 * time.Second):
+		t.Error("Timed out waiting for CheckTx callback")
+	}
+
+	mempoolSize := mempool.Size()
 	for i, c := range GetClients() {
 		mc, ok := c.(client.MempoolClient)
 		require.True(t, ok, "%d", i)