gpg-interface t: extend the existing GPG tests with GPGSM

gpg-interface t: extend the existing GPG tests with GPGSM

Add test cases to cover the new X509/gpgsm support. Most of them
resemble existing ones. They just switch the format to x509 and set the
signingkey when creating signatures. Validation of signatures does not
need any configuration of git, it does need gpgsm to be configured to
trust the key(-chain).
Several of the testcases build on top of existing gpg testcases.
The commit ships a self-signed key for committer@example.com and
configures gpgsm to trust it.

Signed-off-by: Henning Schild <redacted>
Signed-off-by: Junio C Hamano <redacted>