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/random_seed.cc b/aos/testing/random_seed.cc
new file mode 100644
index 0000000..9e7136f
--- /dev/null
+++ b/aos/testing/random_seed.cc
@@ -0,0 +1,17 @@
+#include "aos/testing/random_seed.h"
+
+#include <stdlib.h>
+
+namespace aos {
+namespace testing {
+
+int RandomSeed() {
+  const char *from_environment = getenv("TEST_RANDOM_SEED");
+  if (from_environment != nullptr) {
+    return atoi(from_environment);
+  }
+  return 1;
+}
+
+}  // namespace testing
+}  // namespace aos