Revert CI test timeout. (#7375)

I increased this while messing around with another PR, and
forgot to remove it before merging.

Makefile

@@ -306,4 +306,4 @@ $(BUILDDIR)/packages.txt:$(GO_TEST_FILES) $(BUILDDIR)
 split-test-packages:$(BUILDDIR)/packages.txt
 	split -d -n l/$(NUM_SPLIT) $< $<.
 test-group-%:split-test-packages
-	cat $(BUILDDIR)/packages.txt.$* | xargs go test -mod=readonly -timeout=15m -race -coverprofile=$(BUILDDIR)/$*.profile.out
+	cat $(BUILDDIR)/packages.txt.$* | xargs go test -mod=readonly -timeout=8m -race -coverprofile=$(BUILDDIR)/$*.profile.out