Add new test GIF image to CMake builds too

This should have been done in c2e5749443 (Fix crash when reading
truncated GIF image, 2023-07-02), but was forgotten there -- so do it
now, and add a comment reminding not to forget it the next time.

See #23409.

build/cmake/tests/gui/CMakeLists.txt

@@ -197,6 +197,7 @@ set(TEST_GUI_DATA
     image/toucan_mono_255_255_255.png
     image/width-times-height-overflow.bmp
     image/width_height_32_bit_overflow.pgm
+    image/bad_truncated.gif
     intl/ja/internat.mo
     intl/ja/internat.po
     )

tests/test.bkl

@@ -352,6 +352,10 @@
     <wx-data id="data-images">
         <srcdir>$(SRCDIR)/image</srcdir>
         <dstdir>image</dstdir>
+        <!--
+            Don't forget to update build/cmake/tests/gui/CMakeLists.txt when
+            adding new files here!
+         -->
         <files>
             bitfields.bmp
             bitfields-alpha.bmp