Add StratisCertify tests for snapshot revert

[bgurney-rh]

Related: #280

[bgurney-rh]

The cancel-revert test seems like the only test that I can add right now, as the others would have remnants that clean_up() can't handle right now:

Traceback (most recent call last):
- File "testlib/infra.py", line 126, in clean_up
    f'remnant pools: {", ".join(name for name in remnant_pools)}'
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: sequence item 0: expected str instance, tuple found

...and I had to manually run "stratis fs cancel-revert" for the remnant filesystem snapshot.

[mulkieran]

We'll need to augment the cleanup method with a step to unset all merge fields before we can proceed with the stratis-cli tests.

[bgurney-rh]

I found a way to get the clean_up() method to run, but it took some rework. Re-ordered commits to have this rework done first, before adding the first test.

[bgurney-rh]

Not quite working yet; for some reason, the clean_up() method is not setting "merge" to "false" on the test filesystem created as a result of the "stratis fs snapshot" command.

[bgurney-rh]

Rebasing, putting all of the testlib additions/fixes in the first commit.

[mulkieran]

I have just one requested change. You might as well go on and add a stratis_cert.py test as well and then this should be done.

[mulkieran]

Looks good

[bgurney-rh]

Rebased, to squash clean_up(): destructure MergeScheduled unset check into the main commit for clean_up() changes.

[bgurney-rh]

Rebased.