Since the names of the tinderbox builds apparently aren't reliable (e.g., the latest mozilla-release-linux64-add-on-devel and mozilla-beta-linux64-add-on-devel builds are both Firefox 49)
Don't echo version on Windows, because it hangs Firefox
Also add testing with Firefox Beta (and prior ESR, but commented out for now, since we don't support it and we know it breaks)
Implements the beginnings of unit testing infrastructure using
mocha/chai. The unit tests can be run locally using test/runtests.sh,
although this will need tweaks to run on Windows. They should also run
on commit using Travis-CI.
The unit tests themselves live in test/tests. The index.js file
specifies separate test sets, which can be run individually by calling
test/runtests.sh <testsets>. Right now there is only a single unit
test, but hopefully we'll have more soon...
