[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-full.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+#
+# Run a sequence of gamma tests quietly
+err=0
+
+echo >> pngtest-log.txt
+echo "============ pngvalid-full.sh ==============" >> pngtest-log.txt
+
+echo "Running test-pngvalid-full.sh"
+for gamma in threshold transform sbit 16-to-8 background alpha-mode "transform --expand16" "background --expand16" "alpha-mode --expand16"
+do
+   if ./pngvalid "$@" --gamma-$gamma >> pngtest-log.txt 2>&1
+   then
+      echo "  PASS: pngvalid" "$@" "--gamma-$gamma"
+   else
+      echo "  FAIL: pngvalid" "$@" "--gamma-$gamma"
+      err=1
+   fi
+done
+
+exit $err