tests: revisit tail file deletion tests

[dbluhm]

While working on #2566, I discovered that there are some tests that need attention in aries_cloudagent/revocation/tests/test_routes.py in the TestDeleteTails test case. These tests were not being run (despite the appearance of being run) due to a mistake made in the test class definition: unittest.TestCase instead of asynctest's TestCase was used. As a result, there seems to be some dubious test code. Not dubious in the malicious sense, just unlikely to have ever been in a working state.

We should revisit these tests.