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