Use extreme values for input in convovle tests

The intepolation filter functions can be better tested withe extreme
values, especially given the optimization functions are prone to
overflow signed 16 bit intermediate value when operation order is
wrong.

Change-Id: I712142b0bc1e5969c692c0486a57ffa37c9742b5

test/convolve_test.cc

diff --git a/test/convolve_test.cc b/test/convolve_test.cc
index a6dcc98..2d89429 100644
--- a/test/convolve_test.cc
+++ b/test/convolve_test.cc
@@ -221,8 +221,12 @@
     }
 
     ::libvpx_test::ACMRandom prng;
-    for (int i = 0; i < kInputBufferSize; ++i)
-      input_[i] = prng.Rand8Extremes();
+    for (int i = 0; i < kInputBufferSize; ++i) {
+      if (i & 1)
+        input_[i] = 255;
+      else
+        input_[i] = prng.Rand8Extremes();
+    }
   }
 
   void SetConstantInput(int value) {