GDScript: Only include script file path in test error output

Errors originating in C++ files cause unnecessary diffs whenever the engine is updated
(line number changes, etc.) and would cause CI failures due to different formatting
of the file path on Windows (backslashes, worked around here anyway) and when using
SCU builds (`../scu` insert).

modules/gdscript/tests/scripts/runtime/errors/typed_array_assign_wrong_to_typed.out

@@ -1,7 +1,5 @@
 GDTEST_RUNTIME_ERROR
 >> ERROR
 >> on function: assign()
->> core/variant/array.cpp
->> 222
 >> Method/function failed.
 not ok