tests: It's start-stop-agent which invokes gpg-agent.

* tests/gpgsm/Makefile.am (gpgsm.conf): Don't touch agent-program.
* tests/start-stop-agent: Don't modify gpg.conf and gpgsm.conf.

When start-stop-agent fails, this allows invocation of gpg-agent
by gpg/gpgsm. This enables semihosted build.

• Fixes-commit: 9cd2b58dfb2c21cce64520cf4c726859b583d14e
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>