Extension of logical operator to whole test case

[eeishaan]

It would be nice to have the logical operators introduced in #178 extended to the top most level as well It would look like something like this:

tests_include:
        -  AND:
           - test_interfaces_terse
           - OR:
                 - test_interface_1
                 - test_interface_2

This will help overcome the current limitation of #178 where a user is helpless if logical operation is required between tests which have different commands.