Make test_nvp_sync.SyncLoopingCallTestCase reliable

This patch removes the previous check that verified the mocked
looping call had an exact number of calls, and replaces it with a
simpler check that just verifies that the mock has been called.

As a consequence, this patch also shortens the test duration.

Closes-Bug: 1241969

Change-Id: Ia7bc66e1f2f631e46ddcbe0eacc1d5b2481274cb
Fix a copy&paste typo

Change-Id: I970d3cd51f83c963b04e24294e982a0807d1c1c3
Merge "Fix a copy&paste typo"
Merge "Make test_nvp_sync.SyncLoopingCallTestCase reliable"