Added an onfail argument to wait_until to provide alternate behavior

for a timeout.

src/ZEO/tests/forker.py

@@ -330,13 +330,14 @@ def setUp(test):
     test.globs['wait_disconnected'] = wait_disconnected
 
 
-def wait_until(label, func, timeout=30):
-    now = time.time()
-    giveup = now + 30
+def wait_until(label, func, timeout=30, onfail=None):
+    giveup = time.time() + timeout
     while not func():
-        now = time.time()
         if time.time() > giveup:
+            if onfail is None:
                 raise AssertionError("Timed out waiting for: ", label)
+            else:
+                return onfail()
         time.sleep(0.01)
 
 def wait_connected(storage):