Remove `testportfiller.py` in favor of `build_component.replace_string()`

From what I can tell, the functionality provided by `testportfiller.py` is available from `build_component.py`'s `replace_string` function. Even the code of both is almost identical.

We should replace `testportfiller`'s functionality with `replace_string`'s. (In the long run, we should also redesign how port numbers for sockets are assigned in unit tests.)
