Fixed regression in config-file reading.

This change causes the `wptcommandline.py` code to once again read
the `tests` and `metadata` paths from the `[paths]` section of the
`wptrunner.default.ini` config file or other specified config file.

(In https://github.com/w3c/wptrunner/commit/815882453fec3191d95bfe9777367829151398b8 -- specifically in https://github.com/w3c/wptrunner/commit/815882453fec3191d95bfe9777367829151398b8#diff-409148835eecbf686495abf6ffd37cb5L125
a change was made that regressed config-file reading in such a way as to
cause the code to just ignore any `tests` and `metadata` paths specified in
the `[paths]` section of the `wptrunner.default.ini` config file or other
specified config file.)