tests: Honor environment variable 'TMP'.
This fixes problems with long socket names, e.g. when doing distcheck.
- tests/gpgscm/tests.scm (path-join): New function.
(with-temporary-working-directory): Honor 'TMP'.
(make-temporary-file): Likewise.
- tests/migrations/Makefile.am (TMP): Default to '/tmp'.
(TESTS_ENVIRONMENT): Set 'TMP'.
- tests/openpgp/Makefile.am (TMP): Default to '/tmp'.
(TESTS_ENVIRONMENT): Set 'TMP'.
- Signed-off-by: Justus Winter <justus@g10code.com>