Put mtree test files into a subdirectory.

Kyua 0.11 points TMPDIR to the test's work directory, and atf_check creates
auxiliary files in TMPDIR. This confuses a couple of mtree tests that were
using the work directory's root to validate the contents of the directory.

Fix the two affected tests by creating an auxiliary directory to use for
the mtree tests. (Kyua should probably do this on its own; filed bug #133
upstream to take a look at this.)