These tests used a hardcoded vote with a hardcoded digest. That
vote didn't have any pr lines, and so it is now invalid. I've
adjusted the testing code so that it mocks the signature-checking,
so that we can more easily change the contents of the vote.
This is a big-ish patch, but it's very straightforward. Under this
clang warning, we're not actually allowed to have a global variable
without a previous extern declaration for it. The cases where we
violated this rule fall into three roughly equal groups:
* Stuff that should have been static.
* Stuff that was global but where the extern was local to some
other C file.
* Stuff that was only global when built for the unit tests, that
needed a conditional extern in the headers.
The first two were IMO genuine problems; the last is a wart of how
we build tests.
