Refactor MiqAeExportSpec to decrease test run time

MiqAeExportSpec was our slowest spec, and these refactorings should turn the 30+ seconds that it used to take into about a second. Running all of the changed files (with some old tests that are still setting too much stuff up) takes about 4 seconds, so I'd consider this a significant speed up.

I also noticed that it was attempting to call to_export_xml on instances of CustomButton, but that method does not exist at all on CustomButton. It's implemented here as a placeholder, as I don't know enough about what should or should not be exported from a CustomButton.