run-tests: make time field optional for xunit report

We're going to use XUnit to list tests, and we don't have a time field in that
case.

tests/run-tests.py

@@ -2016,12 +2016,16 b' class TextTestRunner(unittest.TextTestRu'
         for tc in result.successes:
             t = doc.createElement('testcase')
             t.setAttribute('name', tc.name)
-            t.setAttribute('time', '%.3f' % timesd[tc.name])
+            tctime = timesd.get(tc.name)
+            if tctime is not None:
+                t.setAttribute('time', '%.3f' % tctime)
             s.appendChild(t)
         for tc, err in sorted(result.faildata.items()):
             t = doc.createElement('testcase')
             t.setAttribute('name', tc)
-            t.setAttribute('time', '%.3f' % timesd[tc])
+            tctime = timesd.get(tc)
+            if tctime is not None:
+                t.setAttribute('time', '%.3f' % tctime)
             # createCDATASection expects a unicode or it will
             # convert using default conversion rules, which will
             # fail if string isn't ASCII.