[QA] Initial setup for MultimediaViewer browser tests

[Jeff Hall]

FYI, just so that QA-minded folks know it exists, Aaron Arcos and I 
paired this afternoon to setup initial browser tests for the 
MultimediaViewer extension[1]

Since Aaron and I are both criminally lazy, we copied liberally from 
other WMF repos that already have browser tests.  Aaron is planning to 
continue working on this patchset in order to get some good end-to-end 
test coverage for MultimediaViewer.

- Jeff


[1] https://gerrit.wikimedia.org/r/#/c/107517/