tests: GPG: Explicitly check that the crypto ops have finished

This is meant to improve the readability of the error messages during
eventual test failures.

Also unify the form of the check that we use among all of these tests.
Consistency is good.

Change-Id: I638607c0742a295cf29b61a1434d313e7618dc8a
CCBUG: 373374
Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1401716