* tests/gpgscm/ffi.c (ffi_init): Rename 'mkdtemp'.
* tests/gpgscm/tests.scm (mkdtemp): New function that uses a sensible
location and template if no arguments are given.
(with-temporary-working-directory): Simplify accordingly.
(make-temporary-file): Likewise.
* tests/openpgp/run-tests.scm (run-tests-parallel-isolated): Likewise.
(run-tests-sequential-isolated): Likewise.