tests: clean up even more direct `python` calls with $PYTHON
This time ones that are prefixed with =, ", ', or `. This appears to
be the last of them.
Differential Revision: https://phab.mercurial-scm.org/D14
contrib: widen the "don't use `python`" net a little
I'm still cleaning this up, but it's easier to do in bite-size chunks
like this than all at once. The negative lookahead avoids one false
positive category from some output related to finding Subversion
bindings.
Differential Revision: https://phab.mercurial-scm.org/D13