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
(cherry picked from commit 4e311e66deb93b459527bd5e9f203720a8e28463)
Merge "Make test_nvp_sync.SyncLoopingCallTestCase reliable" into stable/havana