Tidy up _test_driver
Tidy up _test_driver

Add header comments to explain what it does.
Do not make multiple calls to _test_driver in any test.
Rename test_combinations and have it call _test_driver only once.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

stratis-storage/stratisd,stratis-storage/stratisd,stratis-storage/stratisd