tests: Make agent spawning more robust.

* tests/gpgsm/Makefile.am (gpgsm.conf): Add agent-program directive.
* tests/start-stop-agent: Update agent-program directive.

Update the 'agent-program' configuration directive to point to the
same agent that we are starting. Previously, it was possible that a
different agent was started if 'make check' was run with a different
PATH.

• Signed-off-by: Justus Winter <justus@g10code.com>