Test \gresetgregpath

[rpspringuel]

Instead of having an alias which translates into a copied file, the test is modified to borrow the gabc file from another test using the new \gresetgregpath mechanism.

Corresponds to gregorio-project/gregorio#1440

[rpspringuel]

This does not test the kpse search implementation because testing that would require setting TEXINPUTS for a test. As there is no way of doing that from within a test, it would require some modification to the testing script itself. @henryso, if you have an idea as to how to do that, I'm all ears.

[henryso]

This seems to be a really specialized thing. Maybe create this as a scripted test?

[rpspringuel]

Possibly. Unlike the other scripted tests, however, this would have to run lualatex, not just gregorio. I'm not sure that it would be necessary to compare the output, however. The point of the test would just be to see if lualatex could find the gabc file when TEXINPUTS is suitably defined.