selftests: ublk: move test temp files into a sub directory

selftests: ublk: move test temp files into a sub directory

Create and use a temporary directory for the files created during
test runs. If TMPDIR environment variable is set use it as a base
for the temporary directory path.
TMPDIR=/mnt/scratch make run_tests
and
TMPDIR=/mnt/scratch ./test_generic_01.sh
will place test directory under /mnt/scratch

Signed-off-by: Alexander Atanasov <alex@zazolabs.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>