Disable flaky TestCommittedAtMigratorUnknownCommits test
Created by: eseliger
Build failure: https://buildkite.com/sourcegraph/sourcegraph/builds/94472#cb61f188-36ed-40e6-a7ba-13310f67e6ce
Created by: eseliger
Build failure: https://buildkite.com/sourcegraph/sourcegraph/builds/94472#cb61f188-36ed-40e6-a7ba-13310f67e6ce