[WebDriver BiDi] Add tests for session.end #63255

The WPT WebDriver BiDi client supports session.end, but webdriver/tests/bidi/session/ currently has no conformance coverage for the command.

The WebDriver BiDi specification requires session.end to:

1. end the current session;
2. return an empty success result;
3. send the response before beginning session cleanup; and
4. clean up the ended session, including closing all of its WebSocket connections.

Tests should cover:

• returning an empty success result and subsequently closing the command's
  WebSocket connection;
• closing every WebSocket connection associated with the ended session, not
  only the connection that sent session.end; and
• ensuring asynchronous cleanup of the ended session cannot close or otherwise
  invalidate a replacement session created after the success response.

Specification:
https://w3c.github.io/webdriver-bidi/#command-session-end