Silence warnings when passing invocations

When passing invocations that return values, you can end up with compiler
warnings that you're not using the result (for example, when verifying that an
`init` method is called). This isn't ideal, and leads to having to add ugly
`#pragma clang` definitions in your test file. Instead, we can use the C99
`_Pragma()` function to disable these warnings inside the macros themselves.