[test] Use generator to accelerate test runner startup.

This saves >1s for mjsunit and >10s for test262.

Review URL: https://codereview.chromium.org/1174923002

Cr-Commit-Position: refs/heads/master@{#28904}

tools/testrunner/local/execution.py

@@ -219,23 +219,20 @@ class Runner(object):
   def _RunInternal(self, jobs):
     pool = Pool(jobs)
     test_map = {}
-    # TODO(machenbach): Instead of filling the queue completely before
-    # pool.imap_unordered, make this a generator that already starts testing
-    # while the queue is filled.
-    queue = []
-    queued_exception = None
+    queued_exception = [None]
+    def gen_tests():
       for test in self.tests:
         assert test.id >= 0
         test_map[test.id] = test
         try:
-        queue.append([self._GetJob(test)])
+          yield [self._GetJob(test)]
         except Exception, e:
           # If this failed, save the exception and re-raise it later (after
           # all other tests have had a chance to run).
-        queued_exception = e
+          queued_exception[0] = e
           continue
     try:
-      it = pool.imap_unordered(RunTest, queue)
+      it = pool.imap_unordered(RunTest, gen_tests())
       for result in it:
         if result.heartbeat:
           self.indicator.Heartbeat()
@@ -257,8 +254,8 @@ class Runner(object):
         # some files might still be open.
         print "Deleting perf test data due to db corruption."
         shutil.rmtree(self.datapath)
-    if queued_exception:
-      raise queued_exception
+    if queued_exception[0]:
+      raise queued_exception[0]
 
     # Make sure that any allocations were printed in predictable mode (if we
     # ran any tests).