fdct8x8_test: fix unsigned overflow

the difference between src and dst will be signed, the error will be
unsigned.
quiets -fsanitize=integer:
unsigned integer overflow: 4294967295 * 4294967295

Change-Id: I580813093ee46284fde7954520dfcb1188f79268

test/fdct8x8_test.cc

diff --git a/test/fdct8x8_test.cc b/test/fdct8x8_test.cc
index 0c081ee..29f2158 100644
--- a/test/fdct8x8_test.cc
+++ b/test/fdct8x8_test.cc
@@ -425,10 +425,10 @@
 
       for (int j = 0; j < kNumCoeffs; ++j) {
 #if CONFIG_VP9_HIGHBITDEPTH
-        const uint32_t diff =
+        const int diff =
             bit_depth_ == VPX_BITS_8 ? dst[j] - src[j] : dst16[j] - src16[j];
 #else
-        const uint32_t diff = dst[j] - src[j];
+        const int diff = dst[j] - src[j];
 #endif
         const uint32_t error = diff * diff;
         EXPECT_GE(1u << 2 * (bit_depth_ - 8), error)
@@ -511,10 +511,10 @@
 
       for (int j = 0; j < kNumCoeffs; ++j) {
 #if CONFIG_VP9_HIGHBITDEPTH
-        const uint32_t diff =
+        const int diff =
             bit_depth_ == VPX_BITS_8 ? dst[j] - ref[j] : dst16[j] - ref16[j];
 #else
-        const uint32_t diff = dst[j] - ref[j];
+        const int diff = dst[j] - ref[j];
 #endif
         const uint32_t error = diff * diff;
         EXPECT_EQ(0u, error)