Add test for partial inverse transforms

To make sure the results they produce to match their full versions

Change-Id: Ib00b036fecf43fe20af716acfc580af18ef87757
diff --git a/test/acm_random.h b/test/acm_random.h
index de94186..496dae3 100644
--- a/test/acm_random.h
+++ b/test/acm_random.h
@@ -26,6 +26,11 @@
   void Reset(int seed) {
     random_.Reseed(seed);
   }
+  uint16_t Rand16(void) {
+    const uint32_t value =
+        random_.Generate(testing::internal::Random::kMaxRange);
+    return (value >> 16) & 0xffff;
+  }
 
   uint8_t Rand8(void) {
     const uint32_t value =