Add report time to item model

This allows reporters to include the enqueue/dequeue time of an
item.  The item model already hase enqueue/dequeue times which we
use when reporting status, however, a reporter runs right before
the item is dequeued.  So we need one more time value which
corresponds to the start of the reporting phase -- thus report_time
in this patch.

Test coverage for this is added in change
Ida0cdef682ca2ce117617eacfb67f371426a3131.

Change-Id: I093626e098b7ce2deea2b0c25265cb48d38712ad

zuul/model.py

@@ -2125,6 +2125,7 @@ class QueueItem(object):
         self.item_ahead = None
         self.items_behind = []
         self.enqueue_time = None
+        self.report_time = None
         self.dequeue_time = None
         self.reported = False
         self.reported_enqueue = False
@@ -2165,6 +2166,7 @@ class QueueItem(object):
         self.current_build_set.removeBuild(build)
 
     def setReportedResult(self, result):
+        self.report_time = time.time()
         self.current_build_set.result = result
 
     def debug(self, msg, indent=0):