Add missing colr box in test_12x34_gainmap_tmap

Generated with
https://github.com/AOMediaCodec/libavif/blob/8d1c71906320c9a161d661f84a7028bd2348cc6d/tests/gtest/avifgainmaptest.cc#L177

Change-Id: Ifa352bc376118521de9e49b2d01a045fc23aa7d2
diff --git a/tests/avifinfo_test_12x34_gainmap_tmap.avif b/tests/avifinfo_test_12x34_gainmap_tmap.avif
index 1de29b5..e6c170e 100644
--- a/tests/avifinfo_test_12x34_gainmap_tmap.avif
+++ b/tests/avifinfo_test_12x34_gainmap_tmap.avif
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