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.
# See http://EditorConfig.org for the specification
root = true
[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true
end_of_line = lf
[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true
end_of_line = lf
[*.t]
indent_size = 2
indent_style = space
trim_trailing_whitespace = false
end_of_line = lf