Only send last message if test still running (#939)

Fix the "Unknown test UID" message that appears in the web gui after any
test has completed. This is caused by the last test record containing
information about a test that has completed. Tests that complete are no
longer in the TEST_INSTANCES list, so the web gui can't retrieve any
more information about it.

Author: jdve

openhtf/output/servers/station_server.py

@@ -280,8 +280,13 @@ def _publish_test_state(cls, test_state_dict, message_type):
     cls._last_message = message
 
   def on_subscribe(self, info):
-    """Send the more recent test state to new subscribers when they connect."""
-    if self._last_message is not None:
+    """
+    Send the more recent test state to new subscribers when they connect,
+    unless the test has already completed.
+    """
+    test, _ = _get_executing_test()
+
+    if self._last_message is not None and test is not None:
       self.send(self._last_message)