time: skip TestLongAdjustTimers on plan9 (too slow)

The TestLongAdjustTimers test has been consistently timing out
after 60 seconds on plan9-arm. Skip the test for plan9, as it
is already skipped for being too slow on android and ios.

Fixes golang#74921

Change-Id: Icc32e902cecd2e98971a898373fe8346b179437d
Reviewed-on: https://go-review.googlesource.com/c/go/+/693955
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Mark Freeman <markfreeman@google.com>

Author: millerresearch

src/time/tick_test.go

@@ -151,7 +151,7 @@ func TestTickerResetLtZeroDuration(t *testing.T) {
 }
 
 func TestLongAdjustTimers(t *testing.T) {
-	if runtime.GOOS == "android" || runtime.GOOS == "ios" {
+	if runtime.GOOS == "android" || runtime.GOOS == "ios" || runtime.GOOS == "plan9" {
 		t.Skipf("skipping on %s - too slow", runtime.GOOS)
 	}
 	t.Parallel()