devicemapper: Fix tests on fedora

If /dev/loop-control exists on the system running the test then
ioctlLoopCtlGetFree() will be called, but if not it won't.  It does
not exist in the standard docker build environment, so the tests
currently require this to not be called. This makes it instead
optional, allowing the tests to run on e.g. Fedora.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #4056 from alexlarsson/fix-devmapper-test

devicemapper: Fix tests on fedora