lib/cocoa: intro message box example in cocoa

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/cocoa: intro an example with extern Objective-C classes

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: add option to skip tests per OS

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: add OS skip list for Linux and cocoa

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>