Add conformance scenario: logging/setLevel and log notifications

[elang2]

The logging utility (logging/setLevel and log message notifications) has no conformance scenario. The spec defines:

• Server MUST accept logging/setLevel with a valid severity level
• Server SHOULD only emit log notifications at or above the configured level
• Valid levels: debug, info, notice, warning, error, critical, alert, emergency

Proposed checks:

• Setting level to "error" stops debug/info/warning notifications
• Setting level to "debug" allows all notifications through
• Invalid level values are rejected appropriately

I'll submit a PR for this.