PSA fission tests by default and shifting tests

[Joel Maher]

Fission is default for Firefox desktop, and that means all our tests should be running in fission mode.  A lot of great work was done over the last few years to get tests stable on fission (or fission stable with our tests), primarily on linux.  There is a push to get this on windows and osx as well being tracked in this meta bug:

https://bugzilla.mozilla.org/show_bug.cgi?id=1750399

There are a few test suites left to migrate which require more coordination and investigation, but for the most part tests are running in both e10s (!fission) and fission. 

Later this week we plan to start turning off the duplicate tests on windows and osx.  The plan is to leave duplication for linux opt/debug/asan until android/geckoview is running fission by default.

Once we are cleaned up on desktop, we will work to shift from -fis tags on treeherder to -nofis tags. 

Thanks to all who have helped so far!  If you have any questions or concerns reach out to me or comment in the bug.