devicemapper: Remove non-general tests

Now that we have the generic graphtest tests that actually tests
the driver we can remove the old mock-using tests. Almost all of
these tests were disabled anyway, and the four remaining ones
didn't really test much while at the same time being really
fragile and making the rest of the code more complex due to
the mocking setup.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: Remove sys.go mocking framework

This is no longer needed, as the tests are gone.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #5842 from alexlarsson/dm-cleanup-tests

 devicemapper: Remove non-general tests