tests: ensure `$PYTHON` is quoted for Windows
Global installs of python3 go into "Program Files", and tons of tests fail with
mysterious errors if this isn't quoted. Most of this is a followup to
0826d684a1b5, but a some of these were existing issues. Shebang lines are
ignored because quoting breaks direct execution- these will need to be launched
indirectly with the quoted `$PYTHON` command.
Differential Revision: https://phab.mercurial-scm.org/D10633
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