Fix UBSAN/IOC errors

1. test/dct16x16_test.cc
2. test/dct32x32_test.cc
3. test/fdct8x8_test.cc

BUG=webm:1225

Change-Id: I9c9315fbd65ddb3b44f688e01ba265fd22192198

test/dct16x16_test.cc

@@ -373,10 +373,10 @@ class Trans16x16TestBase {
 
       for (int j = 0; j < kNumCoeffs; ++j) {
 #if CONFIG_VP9_HIGHBITDEPTH
-        const uint32_t diff =
+        const int32_t diff =
             bit_depth_ == VPX_BITS_8 ?  dst[j] - src[j] : dst16[j] - src16[j];
 #else
-        const uint32_t diff = dst[j] - src[j];
+        const int32_t diff = dst[j] - src[j];
 #endif
         const uint32_t error = diff * diff;
         if (max_error < error)

test/dct32x32_test.cc

@@ -154,10 +154,10 @@ TEST_P(Trans32x32Test, AccuracyCheck) {
 
     for (int j = 0; j < kNumCoeffs; ++j) {
 #if CONFIG_VP9_HIGHBITDEPTH
-      const uint32_t diff =
+      const int32_t diff =
           bit_depth_ == VPX_BITS_8 ? dst[j] - src[j] : dst16[j] - src16[j];
 #else
-      const uint32_t diff = dst[j] - src[j];
+      const int32_t diff = dst[j] - src[j];
 #endif
       const uint32_t error = diff * diff;
       if (max_error < error)

test/fdct8x8_test.cc

@@ -458,7 +458,7 @@ class FwdTrans8x8TestBase {
         coeff_r[j] = static_cast<tran_low_t>(round(out_r[j]));
 
       for (int j = 0; j < kNumCoeffs; ++j) {
-        const uint32_t diff = coeff[j] - coeff_r[j];
+        const int32_t diff = coeff[j] - coeff_r[j];
         const uint32_t error = diff * diff;
         EXPECT_GE(9u << 2 * (bit_depth_ - 8), error)
             << "Error: 8x8 DCT has error " << error