test-gpg: start gpg-agent under control of the test runner
GnuPG v2 automatically starts gpg-agent. We should kill the daemon process.
test-gpg: make temporary copy of GNUPGHOME
GnuPG v2 will convert v1 secret keys and create a socket under $GNUPGHOME.
This patch makes sure no state would persist.
We no longer need to verify trustdb.gpg, which was added by
aae219a99a6e.