Try to fix test_signal breakages on Linux due to r61687. It appears that at

least two of the linux build bots aren't leaving zombie processes around for
os.waitpid to wait for, causing ECHILD errors. This would be a symptom of a bug
somewhere, but probably not in signal itself.

Lib/test/test_signal.py

@@ -44,9 +44,11 @@ class InterProcessSignalTests(unittest.TestCase):
         """Wait for child_pid to finish, ignoring EINTR."""
         while True:
             try:
-                pid, status = os.waitpid(child_pid, 0)
-                return status
+                os.waitpid(child_pid, 0)
+                return
             except OSError as e:
+                if e.errno == errno.ECHILD:
+                    return
                 if e.errno != errno.EINTR:
                     raise