Reserve test_files extensions starting with grm

[ratmice]

This reserves test_files from the %grmtools which have a file extension beginning with .grm
This is regarding #595 (comment)

So we can delay #593 to a future release, and not hold this one up on that, but do so compatibly.

I tested this manually by setting the test_files section of calc_ast
from test_files: "input*.txt" to test_files: "input*", then adding a calc_ast/src/input.grmfoo file,
Then manually ran cargo build and nimbleparse src/calc.{l,y}

This succeeded when the input.grmfoo file was gone, and failed with the appropriate error when it was there.