add Chrome/Firefox interop tests

[fippo]

Description

adds interoperability tests between Chrome unstable and Firefox Nightly. These tests are designed to run nightly as github actions. Github provides a "free" budget of 2000 minutes per month. In the current configuration with four tests (a matrix of Chrome/Firefox) that each take around a minute the nightly runs have a "cost" of 120 minutes per month.

The tests do not rely on a signaling server, instead use the mocha-based test that is controlling the individual selenium webdriver instances to act as a signaling channel which exchanges offers and answers with candidates.
This pattern is described in the testbed repository
https://github.com/fippo/testbed
from 2016, this is a "more modern" take on the same subject.

To run the tests locally,

• npm run mocha test/interop/connection

Running them with Chrome/Firefox unstable requires Linux for the integration with travis-multirunner and setting the

• BROWSER_A
• BROWSER_B
• BVER environment variables.

Purpose
Getting rid of the dependency on apprtc in tests while keeping a minimum level of interoperability tests

[fippo]

I've run those tests for ~2 weeks now and they were surprisingly reliable (unlike the samples tests...)
Adding Safari to the mix is possible (but that has a minute multiplier of 10) too but I'd rather run the basic case that we want to replace apprtc first for a month or two before getting more ambitious.

One of the tricky things is that chromedriver normally has a version check which allow it to operate +- 1 release of the chrome version. This broke when Chrome Unstable went to M108 and required adding the disable-build-check flag.
It might actually be useful to extend the chromedriver flag to specify a range of 2 as an argument instead of completly disabling the check

[KaptenJansson]

LGTM overall but with a disclaimer: it's been a while since I worked on this but I trust you that it works.

NIT: Please add copyright headers on each file.

[alvestrand]

Where do test failures show up?

[fippo]

They will show up on https://github.com/webrtc/samples/actions which can be filtered.
I plan to add a "send email" functionality once running this for a month shows it is solid enough that a failure justifies nagging people ;-)