Merge branch 'st/lib-gpg-kill-stray-agent'

Some versions of GnuPG fails to kill gpg-agent it auto-spawned
and such a left-over agent can interfere with a test.  Work it
around by attempting to kill one before starting a new test.

* st/lib-gpg-kill-stray-agent:
  t: lib-gpg: flush gpg agent on startup