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>