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/aos/testing/BUILD b/aos/testing/BUILD
index 8656d23..a8dce04 100644
--- a/aos/testing/BUILD
+++ b/aos/testing/BUILD
@@ -72,3 +72,15 @@
   ],
   testonly = True,
 )
+
+cc_library(
+  name = 'random_seed',
+  visibility = ['//visibility:public'],
+  srcs = [
+    'random_seed.cc',
+  ],
+  hdrs = [
+    'random_seed.h',
+  ],
+  testonly = True,
+)