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;
```

Is there any reason we couldn't have just used the existing method? i.e. why do we need to check on the extension here when we end up looking within the file data in the `disambiguate_tst` method anyway?