GitLab

Refactors the migration tests to no longer reuse the same
StatusChangeChecker.

Splits the previous MigrationChecker into two separate classes:
MigrationWaiter and MigrationWatcher.  The waiter is long-lived
and observes migrations.  The waiter exists only while we're
waiting for a migration to complete.

BUG=97780, 95742

Review URL: https://codereview.chromium.org/490643004

Cr-Commit-Position: refs/heads/master@{#292720}