Explain test runner SSH configuration

[yusufhay]

Summary

• Adds guidance for when to leave WPT_SSH_CONNECT empty.
• Explains when hosting teams should use SSH to run tests in a remote or shared hosting environment.
• Adds safety notes for test directories, database access, SSH options, and private keys.

Closes #65.

Testing

• Documentation-only change.
• Verified the change is limited to tests.md and adds one focused section after the environment variable example.

[vairafiq]

Nice, clear addition. The distinction between leaving WPT_SSH_CONNECT empty for same server runs versus using it for a remote target is well explained, and the security guidance (dedicated test account and database, protecting the .env holding WPT_SSH_PRIVATE_KEY_BASE64, and not targeting production) is exactly right for this context. It reads consistently with the surrounding tests.md section. LGTM from a docs perspective.