[test] Align numfuzz default flags with standard runner

In order to deprecate different default flags, this sets the flag
passed on the standard runner now also on numfuzz.

No-Try: true
Bug: v8:12177
Change-Id: I3fb6872643f5bfad71362f22a804d22907641c84
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3143992Reviewed-by: Almothana Athamneh <almuthanna@chromium.org>
Commit-Queue: Almothana Athamneh <almuthanna@chromium.org>
Auto-Submit: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76679}

tools/testrunner/num_fuzzer.py

@@ -124,7 +124,7 @@ class NumFuzzer(base_runner.BaseTestRunner):
 
   def _runner_flags(self):
     """Extra default flags specific to the test runner implementation."""
-    flags = ['--no-abort-on-contradictory-flags']
+    flags = ['--no-abort-on-contradictory-flags', '--testing-d8-test-runner']
     if self.infra_staging:
       flags.append('--no-fail')
     return flags