run-tests: fix whitelist/blacklist with directories on Windows
The file name is resolved with `os.path.relpath()` in the `Test` constructor,
which yields `\` on Windows. That doesn't match the `/` separator when using
MSYS tools to build the list, and it isn't obvious that this is the problem
because directory separators can mostly be used interchangeably. The
`--test-list` argument already seems to be properly handled.
cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh="$PYTHON" "$RUNTESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF