Add PNG8 to list of formats to test in tests/validate.h

diff --git a/tests/validate.h b/tests/validate.h
index 29472ee..a45ed60 100644
--- a/tests/validate.h
+++ b/tests/validate.h
@@ -304,6 +304,7 @@
     { "PJPEG", UndefinedCompression, 0.003 },
     { "PLASMA", UndefinedCompression, 0.0 },
     { "PNG", UndefinedCompression, 0.0 },
+    { "PNG8", UndefinedCompression, 0.0 },
     { "PNG24", UndefinedCompression, 0.0 },
     { "PNG32", UndefinedCompression, 0.0 },
     { "PNM", UndefinedCompression, 0.0 },