tests: Add a pre-signed-pull.sh test

tests: Add a pre-signed-pull.sh test

I'm thinking about adding an implementation of ed25519 signatures
with OpenSSL (so we can ship the feature with Fedora CoreOS
without requiring an additional library) and in preparation for
that it's essential that we validate that libsodium-generated
signatures and OpenSSL-generated signatures are compatible.

I don't know if they are yet actually, but the goal of this
new test is to add a pre-generated repository with a signed
commit generated by libsodium.

This will catch if e.g. there's ever a change in libsodium,
or if existing libsodium implementation versions (e.g. the
one in Debian) might differ from what we ship here.