Made many assertions on GitLab branches asynchronous
The PostReceive process has always been asynchronous, but until now, it was fast enough that we could in practice ignore that in these tests. GitLab 13.12 made us realize that because of its rate limiting branches API cache. These changes are not enough to make the tests pass with this cache enabled (default 10s timeout is too short), but they should improve the tests reliability anyway.
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