dashboard: increase plan9's GO_TEST_TIMEOUT_SCALE
dashboard: increase plan9's GO_TEST_TIMEOUT_SCALE

Updates golang/go#29801

Change-Id: Ib346d717ae9b8de18666d6ea543d9bdcd2856f3b
Reviewed-on: https://go-review.googlesource.com/c/158537
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/build,golang/build,golang/build,golang/build