[Bug] lingering gpg-agents from test suite

[David Bremner]

Thanks to changes in gnupg 2.1.20, the agent started by the notmuch test
suite is no longer killed by deleting the home directory.

So yay, we get to adapt to changes in gnupg, again.

See also

    https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=862093