This CL ensures that we add the
'--fuzzing' flag to the num_fuzzer script.
Please note that NumFuzzer does not inherit the
StandardTestRunner class but it inherits
BaseTestRunner so we had to override _runner_flags.

Bug: v8:10755
Change-Id: Ifb779ba402106b8f2ce4d0e13090ef2db468a6ae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2335185
Commit-Queue: Almothana Athamneh <almuthanna@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Liviu Rau <liviurau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69241}