Improve the lvm_set_global_config test
Improve the lvm_set_global_config test

We cannot be sure the config is exactly what we set, LVM
(esp. lvmdbusd) can add something it needs for its own.

Also we should make sure the test properly cleans after itself.

rhinstaller/libblockdev,rhinstaller/libblockdev,rhinstaller/libblockdev