tests: (lsns::filter) skip if /proc/self/uid_map is not writable

tests: (lsns::filter) skip if /proc/self/uid_map is not writable

On some platforms, lsns::filter test failed with the following
message:

   +test_mkfds: failed to write to /proc/self/uid_map: -1: Operation
   not permitted

This change is to skip the test case on such platforms.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>