Improve test/regen.dart

The regen script actually only regenerates a few of the test bindings. Most of the bindings aren't affected. The script should be updated to regenerate *all* the bindings.

Unfortunately most of the tests are written in a way that makes it really hard to automatically regenerate the bindings. The configs are hard coded into the test rather than being separate config files, and while failing tests do save their output, the file names are different enough that it's hard to get the regen script to copy that file to the expectation file. So the bulk of the work here is cleaning up the tests to make automation easier.