ENH: Adds last_available_{session, minute} args to DataPortal (#1528)

This allows optionally setting the last available dts in the DataPortal
explicitly. If these args aren't provided, we fall back to inferring
these from the underlying readers, which was the previous behavior.

zipline/testing/fixtures.py

@@ -1265,6 +1265,9 @@ class WithDataPortal(WithAdjustmentReader,
 
     DATA_PORTAL_FIRST_TRADING_DAY = None
 
+    DATA_PORTAL_LAST_AVAILABLE_SESSION = None
+    DATA_PORTAL_LAST_AVAILABLE_MINUTE = None
+
     def make_data_portal(self):
         if self.DATA_PORTAL_FIRST_TRADING_DAY is None:
             if self.DATA_PORTAL_USE_MINUTE_DATA:
@@ -1300,6 +1303,8 @@ class WithDataPortal(WithAdjustmentReader,
                 if self.DATA_PORTAL_USE_MINUTE_DATA else
                 None
             ),
+            last_available_session=self.DATA_PORTAL_LAST_AVAILABLE_SESSION,
+            last_available_minute=self.DATA_PORTAL_LAST_AVAILABLE_MINUTE,
         )
 
     def init_instance_fixtures(self):