Pass name of file being analyzed to find_by_heuristics

Some languages are sensitive to file names in the sense that
different kinds of files contain somewhat different data.
Example: GAP .tst files contain test cases, which add some
extra data compared to regular code, and as a consequence are
not directly interchangeable with regular source code.

Heuristics may need to take this into account, thus may need
to know the name of the file being analyzed.
Add heuristic distinguishing GAP and Scilab .tst files