Add GAP .tst extension plus two sample files; set correct lexer

This adds a fourth extension for the "GAP" language: .tst, which is used for test cases (which use a slightly different set of syntax rules). Currently, such .tst files are mis-detected as SciLabg, see e.g. here:

  https://github.com/gap-system/SingularInterface/search?l=scilab

Indeed, for that project, GitHub thinks it is written in 67.% Scilab and 28.1% GAP -- but all that "Scilab" code is GAP code.

Secondly, it removes the 'lexer: Text only' mark, as my Pygments patch adding support for GAP was merged in April. I also verified that it is live on GitHub, e.g.:
```gap
# This is GAP code
if x = fail then
  Print("Failure\n");
else
  Print("All seems well\n");
fi;
```

I think (@fingolfin Please correct me if I'm wrong) that there could be a conflict on another extension between GAP and Scilab (for example `.g`). In this other conflict, the heuristic rule used here might not apply: only `.tst` files contain `gap>`.