Support test-handler in BLASTSyncEngine and add queue-syncset test

Many sync-engine operations post to the main handler which meant
we couldn't really test any of these code-paths, so this separates
out the handler into a separate member and adds a constructor for
tests.

Then, using this, add a test for queueSyncSet which has been
missing.

Bug: 264536014
Test: atest SyncEngineTests
Change-Id: I4a00017038c9355799b1f79494169037e38b7d40