TESTS: Properly set argv when starting the secrets responder
This was a nasty one. Because argv was unsert, instead of its typical
setting as the program path, parsing the arguments produced garbage
which resulted in uid and gid being set to a random (?) value. As an
effect, the secrets responder was not running as fake-root in tests and
this triggered permission errors in libsss_secrets and only root is able
to set arbitary secrets.
Reviewed-by: Fabiano Fidêncio <firstname.lastname@example.org>