Avoid having to manually maintain the list of excluded File System tests
in the GitLab Docker CI image. Scan the unit test source extracting
those tests marked with SKIP_IF_NOT_ROOT_FOR_REQUIRED_LOOPDEV_FOR_FS()
to automatically construct the setting for the GTEST_FILTER environment
variable.
Closes!105 - Update used btrfs file system commands, new minimum is
btrfs-progs 4.5