test: force spec reporter in test-runner-watch-mode.mjs

In the CI this test generates TAP output that can confuse the
Python test runner. Avoid the problem by not outputting TAP at
all.

Fixes: nodejs#54535

test/parallel/test-runner-watch-mode.mjs

@@ -1,4 +1,4 @@
-// Flags: --expose-internals
+// Flags: --expose-internals --test-reporter=spec
 import * as common from '../common/index.mjs';
 import { describe, it, beforeEach } from 'node:test';
 import { once } from 'node:events';