tests: default to /tmp as the temporary directory
* tests/check.mk (TESTS_ENVIRONMENT): Default TMPDIR to /tmp,
rather than to the working directory; this is more common in
practice, which makes the tests more real-worldish; and it is
often faster. Also, it avoids some problems with NFS cleanups.
* tests/misc/sort-compress: Remove unnecessary code setting TMPDIR.
* tests/misc/sort-compress-proc: Likewise. Do the final sleep
only if TMPDIR is relative, which should be rarely given the
change to TESTS_ENVIRONMENT.