Moving input/regression to tests/ ?

[Patrick McCarty]

Hi,

This is an idea for the far-off future, but IMO it would be nice to
see input/regression migrate to a new tests/ directory that looks
something like this:

  tests
   `- general
   |   `- *.ly
   `- musicxml
       `- *.xml
       `- *.mxl

Since the input/ directory now only contains regression tests, I think
a toplevel tests/ directory would make a lot more sense.

Thoughts or opinions?

Thanks,
Patrick