tests: Clarify UntilEvent.__call__

The code could actually not loop. Still check that deadline is not reached
when entering, as code often uses "now" as initial deadline.
Also, clarify dosctring.

caucase/test.py

@@ -195,11 +195,12 @@ class UntilEvent(object):
 
   def __call__(self, deadline):
     """
-    Sleep until deadline is reached or self._event is set.
-    Raises utils.SleepInterrupt if self._event is set.
+    Sleep until event is set, then
+    - if self.action is ON_EVENT_RAISE, raises utils.SleepInterrupt.
+    - if self.action is ON_EVENT_EXPIRE, deadline time it returned.
     """
     now = datetime.datetime.utcnow()
-    while now < deadline:
+    if now < deadline:
       self._wait_event.set()
       assert self._event.wait(10)
       self._event.clear()