test/blockchain/v0: mitigate test data race (#4886)

Mitigates the below data race. The proper fix involves not fiddling with reactor internals, which needs a rewrite of the test and possible additional reactor infrastructure.

```
==================
WARNING: DATA RACE
Write at 0x00c001118e78 by goroutine 187:
  github.com/tendermint/tendermint/blockchain/v0.TestBadBlockStopsPeer()
      /go/src/github.com/tendermint/tendermint/blockchain/v0/reactor_test.go:234 +0x9d7
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:992 +0x1eb

Previous read at 0x00c001118e78 by goroutine 326:
  [failed to restore the stack]

Goroutine 187 (running) created at:
  testing.(*T).Run()
      /usr/local/go/src/testing/testing.go:1043 +0x660
  testing.runTests.func1()
      /usr/local/go/src/testing/testing.go:1285 +0xa6
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:992 +0x1eb
  testing.runTests()
      /usr/local/go/src/testing/testing.go:1283 +0x527
  testing.(*M).Run()
      /usr/local/go/src/testing/testing.go:1200 +0x2ff
  main.main()
      _testmain.go:112 +0x337

Goroutine 326 (running) created at:
  github.com/tendermint/tendermint/blockchain/v0.(*BlockchainReactor).OnStart()
      /go/src/github.com/tendermint/tendermint/blockchain/v0/reactor.go:118 +0x12c
  github.com/tendermint/tendermint/libs/service.(*BaseService).Start()
      /go/src/github.com/tendermint/tendermint/libs/service/service.go:140 +0x504
  github.com/tendermint/tendermint/blockchain/v0.(*BlockchainReactor).Start()
      <autogenerated>:1 +0x43
  github.com/tendermint/tendermint/p2p.(*Switch).OnStart()
      /go/src/github.com/tendermint/tendermint/p2p/switch.go:225 +0x120
  github.com/tendermint/tendermint/libs/service.(*BaseService).Start()
      /go/src/github.com/tendermint/tendermint/libs/service/service.go:140 +0x504
  github.com/tendermint/tendermint/p2p.StartSwitches()
      /go/src/github.com/tendermint/tendermint/p2p/test_util.go:168 +0x75
  github.com/tendermint/tendermint/p2p.MakeConnectedSwitches()
      /go/src/github.com/tendermint/tendermint/p2p/test_util.go:89 +0x17d
  github.com/tendermint/tendermint/blockchain/v0.TestBadBlockStopsPeer()
      /go/src/github.com/tendermint/tendermint/blockchain/v0/reactor_test.go:209 +0x768
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:992 +0x1eb
==================
panic: BlockStore can only save contiguous blocks. Wanted 149, got 147

goroutine 1259 [running]:
github.com/tendermint/tendermint/store.(*BlockStore).SaveBlock(0xc000ff9cc0, 0xc001997180, 0xc0010c6a00, 0xc0013b3000)
	/go/src/github.com/tendermint/tendermint/store/store.go:276 +0xbc4
github.com/tendermint/tendermint/blockchain/v0.(*BlockchainReactor).poolRoutine(0xc001118d00, 0x107c000)
	/go/src/github.com/tendermint/tendermint/blockchain/v0/reactor.go:355 +0xe90
created by github.com/tendermint/tendermint/blockchain/v0.(*BlockchainReactor).OnStart
	/go/src/github.com/tendermint/tendermint/blockchain/v0/reactor.go:118 +0x12d
FAIL	github.com/tendermint/tendermint/blockchain/v0	11.447s
FAIL
```

blockchain/v0/reactor_test.go

@@ -220,17 +220,23 @@ func TestBadBlockStopsPeer(t *testing.T) {
 	}()
 
 	for {
-		if reactorPairs[3].reactor.pool.IsCaughtUp() {
+		time.Sleep(1 * time.Second)
+		caughtUp := true
+		for _, r := range reactorPairs {
+			if !r.reactor.pool.IsCaughtUp() {
+				caughtUp = false
+			}
+		}
+		if caughtUp {
 			break
 		}
-
-		time.Sleep(1 * time.Second)
 	}
 
 	//at this time, reactors[0-3] is the newest
 	assert.Equal(t, 3, reactorPairs[1].reactor.Switch.Peers().Size())
 
-	//mark reactorPairs[3] is an invalid peer
+	// Mark reactorPairs[3] as an invalid peer. Fiddling with .store without a mutex is a data
+	// race, but can't be easily avoided.
 	reactorPairs[3].reactor.store = otherChain.reactor.store
 
 	lastReactorPair := newBlockchainReactor(log.TestingLogger(), genDoc, privVals, 0)