Substitute load_url_async() when running tests to support direct
callers of load_url_async() who will expect to read a file in a
callback.
Stub out device_handler_discover_context_commit() since it will remove
discover_options from the given discover_context, but the tests will
check the discover_context to count boot_options.
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>