devmapper: Properly restore mocked functions after test

Currently the tests that mocks or denies functions leave this state
around for the next test. This is no good if we want to actually
test the devicemapper code in later tests.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
vfs graphdriver: Make root dir mode 755

This matches the other backends.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
graphdriver: Add generic test framework for graph drivers

This adds daemon/graphdriver/graphtest/graphtest which has a few
generic tests for all graph drivers, and then uses these
from the btrs, devicemapper and vfs backends.

I've not yet added the aufs backend, because i can't test that here
atm. It should work though.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
graphdriver: Fail initialization if supported but got error

If a graphdriver fails initialization due to ErrNotSupported we ignore
that and keep trying the next. But if some driver has a different
error (for instance if you specified an unknown option for it) we fail
the daemon startup, printing the error, rather than falling back to an
unexected driver (typically vfs) which may not match what you have run
earlier.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
grapdriver: Skip tests on non-supported backends

For now this means the btrfs backend is skipped when run
inside make test. You can however run it manually if you want.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #5497 from alexlarsson/generic-driver-tests

graphdriver: Add generic test framework for graph drivers