Reduce threshold for Horz Superres test

A PSNR threshold was set slightly too high in one of the Horz Superres
tests, causing the test to fail for certain configurations. This patch
reduces the threshold slightly.

BUG=aomedia:1602

Change-Id: I0acd824ce24d54ecf0927d5729d4eff677f37bcb

test/horz_superres_test.cc

diff --git a/test/horz_superres_test.cc b/test/horz_superres_test.cc
index 78b874e..007b8af 100644
--- a/test/horz_superres_test.cc
+++ b/test/horz_superres_test.cc
@@ -31,7 +31,7 @@
 const int kBitrate = 40;
 
 // PSNR thresholds found by experiment
-const double kPSNRThresholds[] = { 27.5, 29.9, 21.8 };
+const double kPSNRThresholds[] = { 27.5, 29.9, 21.7 };
 
 typedef struct {
   const char *filename;