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.

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...))
+	}
+}