Fstab tests in own file. Small fixes elsewhere.

Refactoring LowLevelCommands into snapshots.py abandoned.
Added test for mount failure and corrected function mocking

Added class-wide LowLevelCommand mocking for TestSnapshotting
class.
will_delete factored back in to AptBtrfsSnapshot.delete()

It is a messy solution, but it works and I can't be bothered to
change it yet.