Initial overhaul of python integrated tests for tools.

[angelosa]

Initial draft for a simple test pipeline over an handful of selected tools (pngcmp, romcmp, unidasm) as a proof of concept.
Not yet ready for inclusion, needs at very least agreement over:

1. location of this folder, i.e. sounds more plausible to have it on a different repository than "mamedev/mame".
2. same deal for assets, I've currently moved it to regtests root, over the fear that they may be useful for other testing facilities.
3. I've also disabled both chdman and jedutil tests, can't be bothered in making them "pass" given they both tests too much implementation details to my tastes.
4. Variable/module/folder naming can be better, totally open to suggestions here.
5. Currently using google style docstrings for functions.
6. assets expected stdout text files may go boom on checkout if autocrlf is set wrong especially for Windows.