Re-enable the RgbColorTypeWithTrnsBeforePlte test Relax the RgbColorTypeWithTrnsBeforePlte test by allowing the decoded image to have alpha (if libpng 1.6.47 or newer is used).

diff --git a/tests/data/README.md b/tests/data/README.md
index 2a53e0b..4582bdd 100644
--- a/tests/data/README.md
+++ b/tests/data/README.md

@@ -23,8 +23,9 @@
 
 It is of color type 2 (PNG_COLOR_TYPE_RGB) and has a tRNS chunk before a PLTE
 chunk. Since the PNG specification version 1.2 says "the tRNS chunk [...] must
-follow the PLTE chunk, if any", libpng considers the tRNS chunk as invalid and
-ignores it.
+follow the PLTE chunk, if any", libpng 1.6.46 or older considers the tRNS chunk
+as invalid and ignores it. The behavior changed starting with libpng 1.6.47.
+See https://github.com/pnggroup/libpng/blob/libpng16/CHANGES#L6243-L6246.
 
 ### File [circle_custom_properties.avif](circle_custom_properties.avif)
 

diff --git a/tests/gtest/avifreadimagetest.cc b/tests/gtest/avifreadimagetest.cc
index 31d8584..321c99d 100644
--- a/tests/gtest/avifreadimagetest.cc
+++ b/tests/gtest/avifreadimagetest.cc

@@ -82,18 +82,17 @@
 }
 
 // Verify we can read a PNG file with PNG_COLOR_TYPE_RGB and a tRNS chunk
-// before a PLTE chunk. libpng considers the tRNS chunk as invalid and ignores
-// it, so the decoded image should have no alpha.
-// This test is disabled because the behavior seemed to have changed starting
-// with libpng 1.6.47.
+// before a PLTE chunk, with no MSan use-of-uninitialized-value warnings in
+// avifImageRGBToYUV(). libpng 1.6.46 or older considers the tRNS chunk as
+// invalid and ignores it, so the decoded image has no alpha. The behavior
+// changed starting with libpng 1.6.47.
 // See https://github.com/pnggroup/libpng/blob/libpng16/CHANGES#L6243-L6246.
-TEST(PngTest, DISABLED_RgbColorTypeWithTrnsBeforePlte) {
+TEST(PngTest, RgbColorTypeWithTrnsBeforePlte) {
   const ImagePtr image = testutil::ReadImage(
       data_path, "circle-trns-before-plte.png", AVIF_PIXEL_FORMAT_YUV444, 8);
   ASSERT_NE(image, nullptr);
   EXPECT_EQ(image->width, 100u);
   EXPECT_EQ(image->height, 60u);
-  EXPECT_EQ(image->alphaPlane, nullptr);
 }
 
 constexpr size_t kColorProfileSize = 376;