tests: make libmount tests more portable

tests: make libmount tests more portable

git tree on macbook:
...
warning: the following paths have collided (e.g. case-sensitive paths
on a case-insensitive filesystem) and only one from the same
colliding group is in the working tree:

  'tests/expected/libmount/optstr-apply-user-X'
  'tests/expected/libmount/optstr-apply-user-x'

Let's rename the files.

Addresses: https://github.com/util-linux/util-linux/issues/1583
Signed-off-by: Karel Zak <kzak@redhat.com>