tests: avoid referencing stale readdir pointer

tests: avoid referencing stale readdir pointer

The contents of 'struct dirent' are only valid until the next call to
readdir() or closedir(). It is thus invalid to save a pointer to the
'd_name' field. Somehow this hasn't affected the test suite until
recently when FreeBSD 12 started showing use of uninitialized memory
resulting in test failures.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>