Add missing license metadata in tests/data/README (#2467)

tests/data/README.md

@@ -6,6 +6,8 @@
 
 ![](circle-trns-after-plte.png)
 
+License: [same as libavif](https://github.com/AOMediaCodec/libavif/blob/main/LICENSE)
+
 An opaque blue circle on a transparent green background.
 
 It is of color type 2 (PNG_COLOR_TYPE_RGB) and has a tRNS chunk after a PLTE
@@ -15,6 +17,8 @@ chunk.
 
 ![](circle-trns-before-plte.png)
 
+License: [same as libavif](https://github.com/AOMediaCodec/libavif/blob/main/LICENSE)
+
 An opaque blue circle on a transparent green background.
 
 It is of color type 2 (PNG_COLOR_TYPE_RGB) and has a tRNS chunk before a PLTE
@@ -26,6 +30,8 @@ ignores it.
 
 ![](draw_points.png)
 
+License: [same as libavif](https://github.com/AOMediaCodec/libavif/blob/main/LICENSE)
+
 Source: Generated with ImageMagick's `convert` command:
 
 ```
@@ -684,6 +690,8 @@ SDR image with a gain map to allow tone mapping to HDR. The gain map's width and
 
 ![](colors_wcg_hdr_rec2020.avif)
 
+License: [same as libavif](https://github.com/AOMediaCodec/libavif/blob/main/LICENSE)
+
 SDR and HDR (PQ) AVIF images in various colorspaces.
 The files with 'wcg' (wide color gamut) in their name have colors outside of the sRGB color space.
 The files without 'wcg' in their name have sRGB colors, but expressed in various color spaces.