Ensure changes to api.QUOTA_SYNC_FUNCTIONS are restored.

Add a local callable restore_sync_functions() that is
established as an addCleanup() fixture callable,
making use of a copy of the QUOTA_SYNC_FUNCTIONS
dictionary to restore its previous contents in-place.

Change-Id: I7fa69ea91e517121aba0ee22a71b0594591ae308
Closes-Bug: #1329482
Merge "Ensure changes to api.QUOTA_SYNC_FUNCTIONS are restored."