skip timing-sensitive tests on CircleCI

We had too many spurious test failures in the past.
But on a developer machine, the tests don't usually fail because the
system isn't loaded as much.
So, only disable test on CircleCI.

replication/driver/replication_stepqueue_test.go

@@ -14,11 +14,12 @@ import (
 	"github.com/stretchr/testify/assert"
 
 	"github.com/zrepl/zrepl/daemon/logging/trace"
+	"github.com/zrepl/zrepl/util/zreplcircleci"
 )
 
-// FIXME: this test relies on timing and is thus rather flaky
-// (relies on scheduler responsiveness of < 500ms)
 func TestPqNotconcurrent(t *testing.T) {
+	zreplcircleci.SkipOnCircleCI(t, "because it relies on scheduler responsiveness < 500ms")
+
 	ctx, end := trace.WithTaskFromStack(context.Background())
 	defer end()
 	var ctr uint32
@@ -90,6 +91,8 @@ func (r record) String() string {
 // Hence, perform some statistics on the wakeup times and assert that the mean wakeup
 // times for each step are close together.
 func TestPqConcurrent(t *testing.T) {
+	zreplcircleci.SkipOnCircleCI(t, "because it relies on scheduler responsiveness < 500ms")
+
 	ctx, end := trace.WithTaskFromStack(context.Background())
 	defer end()
 

rpc/dataconn/timeoutconn/timeoutconn_test.go

@@ -14,6 +14,7 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"github.com/zrepl/zrepl/util/socketpair"
+	"github.com/zrepl/zrepl/util/zreplcircleci"
 )
 
 func TestReadTimeout(t *testing.T) {
@@ -81,6 +82,8 @@ func TestWriteTimeout(t *testing.T) {
 }
 
 func TestNoPartialReadsDueToDeadline(t *testing.T) {
+	zreplcircleci.SkipOnCircleCI(t, "needs predictable low scheduling latency")
+
 	a, b, err := socketpair.SocketPair()
 	require.NoError(t, err)
 	defer a.Close()

util/zreplcircleci/zreplcircleci.go

@@ -0,0 +1,13 @@
+package zreplcircleci
+
+import (
+	"fmt"
+	"os"
+	"testing"
+)
+
+func SkipOnCircleCI(t *testing.T, reasonFmt string, args ...interface{}) {
+	if os.Getenv("CIRCLECI") != "" {
+		t.Skipf("This test is skipped in CircleCI. Reason: %s", fmt.Sprintf(reasonFmt, args...))
+	}
+}