Run the Tor Browser test suite as part of the Tails test suite

Tails has a quite extensive integration test suite, which includes a number of Tor Browser tests, but we don't run the Tor Browser's own test suite.

I see a few reasons why Tails CI should run the Tor Browser test suite in Tails:

• it cheaply increases test coverage of the Tor Browser as shipped in Tails, hence lowering the chances that we break stuff;
• it gives us an incentive to continuously build Tails ISO images on our CI with Tor Browser alpha/nightly, which in turn would help identify problems (be it actual Tor Browser bugs or changes that break Tails' use case) earlier in the dev process;
• it gives us more options to implement Tor Browser tests: I bet that in some cases, we'd better add new tests to the Tor Browser test suite, so they benefit everyone, instead of in the Tails test suite;
• it widens the collaboration area between Tails and TB teams, by providing more reasons to communicate and keep each other informed of what we're up to :)

A few years ago (already!) boklm and I did some initial experiments and built a proof-of-concept that run great parts of the Tor Browser test suite as part of the Tails test suite (https://labs.riseup.net/code/issues/7066). But this was in the Mozmill + Selenium days, but since #16009 (moved) happened and all the tests were ported to Marionette).

Since then we did not invest the time required to push this forward. We discussed this a bit with Georg at the Tails summit this year and he encouraged me to file a ticket here so we can discuss & plan this. To be clear, we'll have very little ressources at Tails to invest into this project but if the conclusion is that boklm will do most of the work, I'll happily introduce him to our test suite framework and be his primary point of contact on Tails side.