[libpng17] Make tests/pngstest* work with the original Bourne shell

tests/pngstest

@@ -0,0 +1,13 @@
+#!/bin/sh
+#
+# Usage:
+#
+#  tests/pngstest pattern
+#
+# Runs pngstest on all the contrib/pngsuite/[^x]*${pattern}.png files
+# NOTE: pattern is used to name the temporary files pngstest generates
+#
+pattern="$1"
+shift
+exec ./pngstest --strict --tmpfile "${pattern}" --log ${1+"$@"}\
+   "${srcdir}/contrib/pngsuite/"[a-wyz]*${pattern}".png"