tests: quote $PYTHON for Windows
When unquoted, MSYS sees the colon between the drive letter and path as a Unix
path separator and unhelpfully splits on it, feeding only the drive letter as
the command. Much chaos ensues.
I vaguely remember trying to get the test runner to use /letter/path/to/exe
syntax the last time this happened, without success. I doubt a check-code rule
would work, since sometimes it is quoted, and sometimes the quotes are escaped.