Move testing of primitive types from std

[ChrisDenton]

std shouldn't need to depend on libc for platforms that don't require it, but currently one test means it is required.

It was suggested that this test more properly belongs in the libc crate itself, so I've made this PR for your consideration.

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[ChrisDenton]

Oh wait I just noticed an issue. This needs to be added to the libc-test Cargo.toml otherwise it's not actually being run.

[tgross35]

Wow that is deceptive. We should just use tests/ for everything that doesn't need to disable the harness, then put the no-harness tests somewhere else.

[ChrisDenton]

Yeah, that sounds like a good idea. I only noticed because I looked at it in my editor and it helpfully greyed out the code. I couldn't immediately figure out why so I investigated further.