tests: stabilize change for handling not quoting non-empty-directory
The change originated in
cb1329738d64. I suspect the problem is with the
combination of (re) and the '\' to '/' retry on Windows. I've no idea if py3 on
Windows needs the quoting, since it can't even run `hg` with no arguments.
(It's dying somewhere on the ctype declarations when win32.py is imported.)
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