test_runner: add level parameter to reporter.diagnostic

Added a parameter to allow severity-based formatting for
diagnostic messages. Defaults to 'info'.
This update enables better control over message presentation
(e.g., coloring) based on severity levels such as 'info', 'warn',
and 'error'.

Refs: nodejs#55922

lib/internal/test_runner/tests_stream.js

@@ -114,11 +114,12 @@ class TestsStream extends Readable {
     });
   }
 
-  diagnostic(nesting, loc, message) {
+  diagnostic(nesting, loc, message, level = 'info') {
     this[kEmitMessage]('test:diagnostic', {
       __proto__: null,
       nesting,
       message,
+      level,
       ...loc,
     });
   }