#7 Access violation when adding same suite twice

[Altforweilerer]

If you accidently call suite::main().add("SomeName", this); twice for the same test suite, an access violation will occur in the destructor when the program exits because it tries to delete the suite twice. Since this is a hard to find bug, there should be a check when adding a test suite whether it has already been added, and a proper error message should be shown.