test: add tests for authenticate command

- Add 20 tests covering OAUTHBEARER, XOAUTH2, XOAUTH authentication
- Add tests for SASL PLAIN with and without authzid (impersonation)
- Add tests for SASL LOGIN with username/password prompts
- Add tests for error handling, malformed responses, unsupported mechanisms
- Add tests for auth method preference (OAuth > PLAIN > LOGIN)
- Coverage improved: authenticate.js 5.74% -> 95.40%
- Overall coverage: 73.51% -> 75.10%

Author: andris9