avifavmtest:Lower PSNR threshold from 39.0 to 38.1

The lower PSNR was introduced in one of the AVM commits from
https://gitlab.com/AOMediaCodec/avm/-/commit/45332a6c to
https://gitlab.com/AOMediaCodec/avm/-/commit/d57c1e27, inclusive.
I could not track down the commit because avifavmtest started to crash
in tflite in https://gitlab.com/AOMediaCodec/avm/-/commit/6191f463 and
the crash was not fixed until
https://gitlab.com/AOMediaCodec/avm/-/commit/d57c1e27. The crash is
https://gitlab.com/AOMediaCodec/avm/-/issues/673.

Bug: b:421144885
diff --git a/tests/gtest/avifavmtest.cc b/tests/gtest/avifavmtest.cc
index 7d9d78c..6c04f2d 100644
--- a/tests/gtest/avifavmtest.cc
+++ b/tests/gtest/avifavmtest.cc
@@ -44,7 +44,7 @@
             AVIF_RESULT_OK);
 
   // Verify that the input and decoded images are close.
-  EXPECT_GT(testutil::GetPsnr(*image, *decoded), 39.0);
+  EXPECT_GT(testutil::GetPsnr(*image, *decoded), 38.1);
 
   // Forcing an AV1 decoding codec should fail.
   for (avifCodecChoice av1_codec :