[wptrunner] Add a wpewebkit_minibrowser product/browser.
This adds a new browser type to run tests with the WPE WebKit port.
It can be passed an optional --headless parameter to enable
headless testing with WPT. Otherwise wayland is required.
A pre-built nightly bundle can be also automatically downloaded and
installed by passing --install-browser to the WPT runner.
It shares with webkitgtk_minibrowser most of the logic to download
and install the built product.
Documentation is also added and the webkitgtk_minibrowser one updated
accordingly. Unit tests are also added.