Attempt to de-flake the end-to-end alert test
Attempt to de-flake the end-to-end alert test

Instead of depending on the alertmanager's goroutine, manually trigger
the tick() function.

BUG=skia:

Review URL: https://codereview.chromium.org/1002283002

Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot