Add unit test of OTE linear model _apply_global_zernikes()

[mperrin]

As suggested by @kjbrooks in comment on #339:

The only thing that I think might be missing is a test of _apply_global_zernikes() since that is never tested. If you have a small test that you can write for it and to make sure it is working well with _apply_global_hexikes() then I think this would be more complete.