time: skip TestTimerModifiedEarlier on plan9/arm
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 <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

CAFxX/go,cloudflare/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go,golang/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,golang/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go