[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-pngvalid-simple.sh

@@ -0,0 +1,30 @@
+#!/bin/sh
+#
+# Run a sequence of tests quietly, without the slow
+# gamma tests
+err=0
+
+echo >> pngtest-log.txt
+echo "============ pngvalid-simple.sh ==============" >> pngtest-log.txt
+echo "Running test-pngvalid-simple.sh"
+# The options to test are:
+#
+# standard tests with and without progressive reading and interlace
+# size images with and without progressive reading
+# transform tests (standard, non-interlaced only)
+#
+for opts in "--standard" "--standard --progressive-read" \
+   "--standard --interlace" "--standard --progressive-read --interlace" \
+   "--size" "--size --progressive-read" \
+   "--transform"
+do
+   if ./pngvalid "$@" $opts >> pngtest-log.txt 2>&1
+   then
+      echo "  PASS: pngvalid" "$@" $opts
+   else
+      echo "  FAIL: pngvalid" "$@" $opts
+      err=1
+   fi
+done
+
+exit $err