tests/shell: set valgrind's "--vgdb-prefix=" to orignal TMPDIR

tests/shell: set valgrind's "--vgdb-prefix=" to orignal TMPDIR

"test-wrapper.sh" sets TMPDIR="$NFT_TEST_TESTTMPDIR". That is useful, so
that temporary files of the tests are placed inside the test result
data.

Sometimes tests miss to delete those files, which would result in piling
up /tmp/tmp.XXXXXXXXXX files. By setting $TMPDIR, those files are
clearly related to the test run that created them, and can be deleted
together.

However, valgrind likes to create files like
"vgdb-pipe-from-vgdb-to-68-by-thom-on-???"  inside $TMPDIR. These are
pipes, so if you run `grep -R ^ /tmp/nft-test.latest` while
the test is still running (to inspect the results), then the process
hands reading from the pipe.

Instead, tell valgrind to put those files in the original TMPDIR. For
that purpose, export NFT_TEST_TMPDIR_ORIG from "run-tests.sh".

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>