Event testing integration with power control testing.

* register_event fixture implanted in the tests
* modified event testing to collect results after each test run
* polling the listener now uses the wait_for to speed things up in case of success
* changed the format of the output HTML, events are grouped by tests
* and other possible fixup

In previous PR, wait_for was implanted into vm power control in pages/ so this speeds things up too.
wait_for