test suite: lib-gdr: Check that we made patches with gdr

Conveniently, the places where we don't want this, and are
deliberately looking for dgit-self-made patches, are gdr-makepatches7
and mix-it in gdr-subcommands; there, we don't push, so don't end up
checking t-gdr-made-patches.

We also add this check to the appropriate place in gdr-subcommands.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>