Run tests on iOS

[SSheldon]

Support for other architectures was added in e1a9133..2e9192f, so these architectures should also be tested.

This is difficult because Rust's test compile down to an executable, and arbitrary executables can't be run on iOS. rust-lang/rfcs#816 will likely be required to run the existing tests on iOS.

[SSheldon]

Compiling with --test generates code like:

pub mod __test {
    extern crate test;

    #[main]
    pub fn main() -> () {
        test::test_main_static(::std::env::args(), TESTS);
    }

    const TESTS: &'static [test::TestDescAndFn] = &[
        ...
    ];
}

If this was instead compiled to a library and TESTS was public, it'd be trivial to link against that library and write an iOS app test runner.

[SSheldon]

A test app was created in 2e9192f..2f64ab8, and it's already caught bugs! I still have to run the tests on 32-bit ARM but the coding here is complete.