Merge branch 'ch/new-gpg-drops-rfc-1991' into maint

Older GnuPG implementations may not correctly import the keyring
material we prepare for the tests to use.

* ch/new-gpg-drops-rfc-1991:
t/lib-gpg: sanity-check that we can actually sign
t/lib-gpg: include separate public keys in keyring.gpg