[functional testing] Support containerized tor-browser-selenium tests in CI-

[conorsch]

Description

The tor-browser-selenium epic (#3659) involves substantial changes to the functional test suite. During review of #3628, we noticed that the CircleCI logic was falling down due to an unclear interaction with the new containerized test logic.

Some work has already been done to address the issues (30f014a), but it's as yet incomplete. Once the functional test suite has stabilized in the tbb-0.9.0 feature branch, we'll need to revise the CI logic to get the containerized tests passing again.

User Stories

As a developer, I want to trust CI to run the entire test suite, to catch errors prior to merge of code changes.

As a maintainer, I want to ensure that CI remains happy, so we can focus on improving code changes within the project.

[eloquence]

Since we still haven't identified the root cause of the CI failures, we've committed for now to a 4 hour time-boxed investigation as part of the 8/8-8/22 sprint. We can then decide on next steps.

[eloquence]

Resolved via #3715.