Add pixel conversion speed test

Add a tests directory where we can put tests programs that can
be useful during testing. Start with a speed test for the pixel
conversion routines.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2593a64..bdb1824 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -416,6 +416,8 @@
   add_subdirectory(media)
 endif()
 
+add_subdirectory(tests)
+
 
 include(cmake/BuildPackages.cmake)