Miscellaneous pwb improvements

Improve pwb emulation of Python 3, so that it sets __builtins__ and
__package__, doesnt use imp module deprecated in Python 3.4.  These
changes are partial sync with the coverage code from whence it came.

Minor updates to our pwb test so it now works in Python 3.  The
same coverage test is now wildly more complex; beyond pywikibot's
needs.

c5d5b01 removed Site interaction from the library bootstrap logic.
The pwb tests can now be flagged as 'net' only, and not 'site', tests.
The tests now run pre-merge with jenkins.

Move the pwb running functionality out of script_tests into utils.

Bug: T72336
Change-Id: Ic2443c17c9ad3976d654b2dd29201cfc12134322
Merge "Miscellaneous pwb improvements"