Simplify Silence test to remove unnecessary wait.
Simplify Silence test to remove unnecessary wait.

As noted in #2867, there is an unnecessary require.Eventually in a
silence test. This PR addresses that by using a channel to signal that
that the maintenance loop has completed.

Signed-off-by: Joe Blubaugh <59f4c54a340094bd42f9e195f2641ce2b3c58ec3@grafana.com>

GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,prometheus/alertmanager