Revert to snapshot tests may incorrectly pass when the feature is not supported

While testing the revert-to-snapshot support for Ceph https://review.opendev.org/#/c/710334/), an observation was made that the two cinder-tempest-plugin tests that are meant to cover this feature had passed even though the feature was still not supported.

More in details, this DNM patch was executed *before* merging the revert-to-snapshot support in Ceph: https://review.opendev.org/#/c/716919/.

Quoting Brian Rosmaita from that review:

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Patch Set 1:

OK, I think I know what's going on here.

cinder_tempest_plugin.api.volume.test_volume_revert.VolumeRevertTests test_volume_revert_to_snapshot[id-87b7dcb7-4950-4a3a-802c-ece55491846d]

    calls revert_to_snapshot, gets a 202
    waits for volume to be available, which it is since nothing is happening
    waits for snapshot to be available, which it is since nothing is happening
    asserts that the volume size has not changed, which it hasn't, since nothing is happening
    should pass, whether the driver supports revert-to-snapshot or not

test_volume_revert_to_snapshot_after_extended_negative[id-4e8b0788-87fe-430d-be7a-444d7f8e0347]

    checks for a BadRequest being raised
    gets caught at the api layer, driver is never involved
    should pass whether the driver supports revert-to-snapshot or not