[libpng16] Added "tunknown" test and corrected a logic error in

png_handle_unknown() when SAVE support is absent.  Moved the shell test
scripts for contrib/libtests from the libpng top directory to contrib/libtests.
png_handle_unknown() must always read or skip the chunk, if
SAVE_UNKNOWN_CHUNKS is turned off *and* the application does not set
a user callback an unknown chunk will not be read, leading to a read
error, which was revealed by the "tunknown" test.

contrib/libtests/test-pngstest.sh

@@ -0,0 +1,24 @@
+#!/bin/sh
+#
+# Run the simplified API tests
+err=0
+
+echo >> pngtest-log.txt
+echo "============ pngstest.sh ==============" >> pngtest-log.txt
+
+echo "Running test-pngstest.sh"
+for image in ${srcdir}/contrib/pngsuite/*.png
+do
+   for opts in ""
+   do
+      if ./pngstest --strict --log "$@" $opts $image >>pngtest-log.txt 2>&1
+      then
+         echo "  PASS: pngstest $opts $image"
+      else
+         echo "  FAIL: pngstest $opts $image"
+         err=1
+      fi
+   done
+done
+
+exit $err