runtime/trace: skip TestUserTaskSpan upon timestamp error

Change-Id: I030baaa0a0abf1e43449faaf676d389a28a868a3
Reviewed-on: https://go-review.googlesource.com/97857
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Peter Weinberger <pjw@google.com>

src/runtime/trace/annotation_test.go

@@ -50,6 +50,10 @@ func TestUserTaskSpan(t *testing.T) {
 
 	saveTrace(t, buf, "TestUserTaskSpan")
 	res, err := trace.Parse(buf, "")
+	if err == trace.ErrTimeOrder {
+		// golang.org/issues/16755
+		t.Skipf("skipping trace: %v", err)
+	}
 	if err != nil {
 		t.Fatalf("Parse failed: %v", err)
 	}