tests: Add missing variable to gpgsm tests Makefile

* tests/gpgsm/Makefile.am (GPG_AGENT): Set var.  It is used later.

Note that the var is used by the gpgsm.conf target.

• Reported-by: Alon Bar-Lev <alon.barlev@gmail.com>
• Signed-off-by: Werner Koch <wk@gnupg.org>