Fix position_sensor_sim's latched_pot value

I noticed it was changing every cycle, which is inaccurate and was
bothering me.

Change-Id: I288ce04ba43d4907a82f9e6c235357d7467e4dde
diff --git a/frc971/control_loops/BUILD b/frc971/control_loops/BUILD
index 5eeed1d..9dee424 100644
--- a/frc971/control_loops/BUILD
+++ b/frc971/control_loops/BUILD
@@ -59,6 +59,7 @@
 
 cc_library(
   name = 'position_sensor_sim',
+  testonly = True,
   srcs = [
     'position_sensor_sim.cc',
   ],
@@ -69,6 +70,7 @@
     ':queues',
     ':gaussian_noise',
     '//debian:libm',
+    '//aos/testing:random_seed',
   ],
 )