Test runner: Avoid race condition

A recent extension to the test runner introduced support for running
tests in parallel using multi-threading. Following this, the runner
would incorrectly emit the "final report" before all individual test
results.

In order to emit the "final report" at the end of the output stream, the
parent thread would initialize all children and wait for availability of
a "log lock" shared by all children.

According to the documentation on the "threading" module's Lock object
[1]:

> When more than one thread is blocked in acquire() waiting for the state
> to turn to unlocked, only one thread proceeds when a release() call
> resets the state to unlocked; which one of the waiting threads proceeds
> is not defined, and may vary across implementations.

This means the primitive cannot be used by the parent thread to reliably
detect completion of all child threads.

Update the parent to maintain a reference for each child thread, and to
explicitly wait for every child thread to complete before emitting the
final result.

[1] https://docs.python.org/2/library/threading.html#lock-objects

tools/packaging/test262.py

@@ -583,6 +583,7 @@ class TestSuite(object):
           SkipCaseElement.append(SkipElement)
           TestSuiteElement.append(SkipCaseElement)
 
+    threads = []
     if workers_count > 1:
       pool_sem = threading.Semaphore(workers_count)
       log_lock = threading.Lock()
@@ -613,11 +614,13 @@ class TestSuite(object):
         exec_case()
       else:
         pool_sem.acquire()
-        threading.Thread(target=exec_case).start()
+        thread = threading.Thread(target=exec_case)
+        threads.append(thread)
+        thread.start()
         pool_sem.release()
 
-    if workers_count > 1:
-      log_lock.acquire()
+    for thread in threads:
+      thread.join()
 
     if print_summary:
       self.PrintSummary(progress, logname)
@@ -628,9 +631,6 @@ class TestSuite(object):
         print "Use --full-summary to see output from failed tests"
     print
 
-    if workers_count > 1:
-      log_lock.release()
-
     return progress.failed
 
   def WriteLog(self, result):