Explicitly import nxt.sensor sub-modules in tests

It was only working because pytest imports everything in nxt to look for
docstrings.

Make the test self contained.

Author: schodet

tests/test_sensors.py

@@ -15,6 +15,11 @@
 import pytest
 
 import nxt.sensor
+import nxt.sensor.analog
+import nxt.sensor.digital
+import nxt.sensor.generic
+import nxt.sensor.hitechnic
+import nxt.sensor.mindsensors
 from nxt.sensor import Mode, Port, Type