This pull request fixes the flay test TestSkeletonSyncRetrievals. In this test, we first
trigger a sync cycle and wait for it to meet certain expectations. We then inject a new
head and potentially also a new peer, then perform another final sync. The test now
performs the newPeer addition before launching the final sync, and waits a bit for that
peer to get registered. This fixes the logic race that made the test fail sometimes.
Co-authored-by: Guillaume Ballet <3272758+gballet@users.noreply.github.com>
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