xfstests: use a common _filter_test_dir function

xfstests: use a common _filter_test_dir function

Christoph Hellwig suggested that a function similar to the common
"_filter_scratch" function ought to be created to handle filtering
of the TEST_DIR and TEST_DEV variables.  This patch implements that.

The name "_filter_test" seems like it might suggest it does
something different, so I'm calling this one "_filter_test_dir".
This unfortunately makes the "test" and "scratch" functions have
different naming conventions, but I guess we should be accustomed to
that by now (consider "TEST_DIR" and "SCRATCH_MNT").

Signed-off-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>