[test] Fix a rare numfuzz hang up

The numfuzz fuzzer.py has a loop to send a new test after receiving
a result. When all test processors go into stopped state, attempts
of sending new tests return False. That case wasn't handled here
and we kept looping forever.

Bug: v8:13113
Change-Id: Ief2686614d9703fb590400ac3e73b6ac9008c8f6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3891373Reviewed-by: Alexander Schulze <alexschulze@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83188}

tools/testrunner/testproc/fuzzer.py

@@ -245,10 +245,11 @@ class FuzzerProc(base.TestProcProducer):
       i += 1
 
   def _try_send_next_test(self, test):
-    if not self.is_stopped:
     for subtest in self._gens[test.procid]:
       if self._send_test(subtest):
         return True
+      elif self.is_stopped:
+        return False
 
     del self._gens[test.procid]
     return False