tests: Reorder the tests so test guest is created before it's used by mountable test.

This fixes commit 7d976657e6f02ca0b61858f668ae792a70327309.
tests: Add Fedora btrfs phony guest to target list.

This fixes commit 754e8194382a9ed265c7fa172e438049e4eb8389.
tests: mountable: Print error message if guestfs_create fails.