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 <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>