test_runner: set level for coverage threshold diagnostics

Updated coverage threshold checks in  to use the
parameter when calling . Errors now use the
'error' level for red-colored formatting.

This ensures coverage errors are highlighted effectively in the output.

Fixes: nodejs#55922

lib/internal/test_runner/test.js

@@ -1086,7 +1086,7 @@ class Test extends AsyncResource {
           if (actual < threshold) {
             harness.success = false;
             process.exitCode = kGenericUserError;
-            reporter.diagnostic(nesting, loc, `Error: ${NumberPrototypeToFixed(actual, 2)}% ${name} coverage does not meet threshold of ${threshold}%.`);
+            reporter.diagnostic(nesting, loc, `Error: ${NumberPrototypeToFixed(actual, 2)}% ${name} coverage does not meet threshold of ${threshold}%.`, 'error');
           }
         }