Add a skip_test() method to the infrastructure and use it for test scripts that skip...

Add a skip_test() method to the infrastructure and use it for test scripts that skip all or part of their tests based on tool availability or test platform.

git-svn-id: http://scons.tigris.org/svn/scons/trunk@1325 fdb21ef1-2011-0410-befe-b5e4ea1792b1