Add a test for a btrfs error associated with small devices (#1109195)

Related: fed#1109195

Signed-off-by: mulhern <amulhern@redhat.com>
Accept None for btrfs raid levels (#1109195)

Resolves: fed#1109195

None means None was assigned, so accept any default that, e.g., mkfs.btrfs
decides on. This default is hard to predict, as it may vary with the size of
the backing device as well as other factors.

Signed-off-by: mulhern <amulhern@redhat.com>
Move DevicelibsTestCase up to the top level of the testing directory.

It is used by formats_test tests as well as tests in devicelibs_test.
Change all uses appropriately.

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