[tests] suppress debug logs in CI environment + improve calendar symbol test stability (#3941)

## CI Log Suppression

**Two-level approach for clean test output:**

1. **Suppress debug/info logs**: Call `logger.setLogLevel("ERROR")` in
CI to hide verbose logging
2. **Suppress intentional error logs**: Set `mmTestMode` flag and check
it before logging errors that are part of test assertions (e.g., testing
error handling in `git_helper.js` and `server_functions.js`)

This keeps CI output clean and makes genuine failures immediately
visible, while preserving full logging for local development.

**Before:** 1348 log lines with verbose debug/info output  
**After:** 168 log clean lines with only test results

## Calendar Symbol Test Stability

Convert the calendar symbol test from external URL (`calendarlabs.com`)
to existing local mock file (`12_events.ics`). This eliminates CI
timeouts caused by external dependencies and improves test reliability.

The test still validates the same symbol array feature but now runs
faster and deterministically without network dependencies.

js/server_functions.js

@@ -58,7 +58,10 @@ async function cors (req, res) {
 			res.send(data);
 		}
 	} catch (error) {
-		Log.error(error);
+		// Only log errors in non-test environments to keep test output clean
+		if (process.env.mmTestMode !== "true") {
+			Log.error(error);
+		}
 		res.send(error);
 	}
 }