exra/runner/report.py: video.webm is played in frame-by-frame player

Purpose of this commit is to replace direct link to video.webm with new
page that has more advanced interface than what usually built-in browser
player offers. In first iteration I am adding support to go
forward/backward by just one frame. When video is playing with standard
speed, sometimes it's very hard to explain and notice why given test
fails or passes.

Files touched by this pull request:
 - runner: creates new preview.html per directory with tests
 - report.py: is able to generate preview.html using template
 - preview.html: new template with all required javascript
 - testrun.html: modified link to point not into raw video but rather to
   preview.html