time: skip TestTimerModifiedEarlier on plan9/arm

This test is observed to be flaky on the plan9-arm builder.
Skip it on that platform until it can be diagnosed and fixed.

For #50470

Change-Id: If626af426d856c377e00ac5baaca52899456556e
Reviewed-on: https://go-review.googlesource.com/c/go/+/375934
Trust: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: Bryan C. Mills

src/time/sleep_test.go

@@ -7,6 +7,7 @@ package time_test
 import (
 	"errors"
 	"fmt"
+	"internal/testenv"
 	"math/rand"
 	"runtime"
 	"strings"
@@ -531,6 +532,10 @@ func TestZeroTimer(t *testing.T) {
 // Test that rapidly moving a timer earlier doesn't cause it to get dropped.
 // Issue 47329.
 func TestTimerModifiedEarlier(t *testing.T) {
+	if runtime.GOOS == "plan9" && runtime.GOARCH == "arm" {
+		testenv.SkipFlaky(t, 50470)
+	}
+
 	past := Until(Unix(0, 0))
 	count := 1000
 	fail := 0