RPerl::Test::Modes - encapsulate the testing modes

Of all alternatives to the explicit for loop through testing modes, I think none comes close by handyness. I'd therefore propose to keep the loop, but to hide the details in the separate module. This way, the .t file still holds the main code to be tested, and there's flexibility to add mode-specific code (such as, if($types eq 'CPP')) here).

On a separate note, there's some bug in older g++'s on MinGW that fail numeric tests on windows; I attach patch for using doubles vs long doubles.