Integration tests in CI

[alan-george-lk]

This PR does the following:

• Moves all testing into a new tests.yml CI stage
• Adds the ability to run integration tests in CI via a locally-installed LiveKit server
• Adjusts unstable integration tests to improve stability, via handshakes/setup
• A more explicit parallel jobs function for improving CI times (has helped slightly on Windows, for example)
• Consolidates 5 different prior env vars for integration/stress tests down to LIVEKIT_TOKEN_A/B

Some caveats:

• Still runs unit tests on all prior platforms
• Only runs integration tests on Linux x64 and Mac arm64, which matches the Rust SDK repo 1:1 (by design)
  • Has matrix option e2e-testing which is how the Rust SDK defines it, for integration tests