Re-enable writing of "Dates" at the end of the run.

The permissions issues which led to the writing being disabled no longer
exist and not persisting the date list makes b2 unsuitable for use as a
primary implementation.

britney.py

@@ -2649,9 +2649,8 @@ class Britney:
             if self.options.control_files:
                 self.write_controlfiles(self.options.testing, 'testing')
 
-            # write bugs and dates
-            # XXX: disabled, we don't have write permission over there
-            # self.write_dates(self.options.testing, self.dates)
+            # write dates
+            self.write_dates(self.options.testing, self.dates)
 
             # write HeidiResult
             self.write_heidi(self.options.heidi_output)