This commit adds a "echo_server_port" property to `WebContentOptions`.
Additionally, it makes `Application::web_content_options()` return a
mutable reference instead of a const reference so that we can set the
port value from the fixture.
This adds a verbosity option to log the start and end of each test, with
the duration taken for the test. To be able to use this option with our
exisiting verbosity flag, without cluttering stdout with other data, we
add verbosity levels to headless-browser. The level is increased by
providing the -v flag on the command line multiple times.
LibWebView now knows how to launch RequestServer and ImageDecoderServer
without help from the UI, so let's move ownership of these services over
to LibWebView for de-duplication.
