Don't require NSObject for tests

[SSheldon]

Apple's Objective-C runtime includes the definition of NSObject, so it had been used in the tests for convenience. However, NSObject isn't defined by the GNUstep runtime, so we had to add the test_ns_object crate to define it for GNUstep.

The tests don't actually need NSObject though, so with a little bit more work we can make one of the test classes a root class and use it instead.

[SSheldon]

cc @ngrewe, I think this will make maintaining the GNUstep feature easier, let me know if you have any thoughts on this.

[ngrewe]

I like the idea. Do you think it might be a useful to factor this out into a separate crate to share as a dev-dependency for the other Obj-C interop crates?

[SSheldon]

@ngrewe I can only imagine this being useful if all you want is to make sure sending messages is working, and the only crate I can imagine wanting to test that is this one 😄 If you come across any crates where it'd be useful I'm happy to extract it!