tests/shell: export NFT_TEST_BASEDIR and NFT_TEST_TMPDIR for tests

tests/shell: export NFT_TEST_BASEDIR and NFT_TEST_TMPDIR for tests

Let the test wrapper prepare and export two environment variables for
the test:

- "$NFT_TEST_BASEDIR" is just the top directory where the test scripts
  lie.

- "$NFT_TEST_TMPDIR" is a `mktemp` directory created by "run-tests.sh"
  and removed at the end. Tests may use that to leave data there.
  This directory will be used for various things, like the "nft" wrapper
  in valgrind mode, the results of the tests and possibly as cache for
  feature detection.

The "$NFT_TEST_TMPDIR" was already used before with the "VALGRIND=y"
mode. It's only renamed and got an extended purpose.

Also drop the unnecessary first detection of "$DIFF" and the "$SRC_NFT"
variable.

Also, note that the mktemp creates the temporary directory under /tmp.
Which is commonly a tempfs. The user can override that by exporting
TMPDIR.

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