ucd-generate should have tests

[thomcc]

At a minimum we should verify that the things we output compile. This sounds doable-if-tedious. We do this now for some things, in the benchmarks. (It probably should be wider, although I'm not going to say we should try every flag combo or anything -- just ones likely to interact).

Ideally we could go one step further and verify that the data we output matches some reference version of the data. ucd-generate itself could even emit the tests.

[BurntSushi]

In theory I'm okay with this. But I personally don't want to have too much ceremony around it. That is, I would like to keep the barrier to adding new sub-commands relatively small. So tests that don't need to be changed or added to when a new command is added would be a nice win. (e.g., Checking that the output compiles.)