Move unit tests into its own file

[gnzlbg]

This PR moves all unit tests into their own file so that they are not included when pulling the math module via a #[path] import (like the one in compiler builtins, or in a crate that wants to build a cdylib).

[alexcrichton]

I'm not sure what the reason for this is? I personally think it's great to be able to write tests inline and have them executed. You're in general not really supposed to use #[path] to import crates but even if it were used these are always configured out, right?

[gnzlbg]

The compiler-builtin crate uses #[path], and since people forget to #[cfg(test)] their tests out (e.g. https://github.com/rust-lang-nursery/libm/blob/de8ed2dbc0fe89e55369cc8eba706b6c3d8086be/src/math/sin.rs#L82), this means that #[test] end up always compiled into compiler-builtins unconditionally.

[gnzlbg]

Alternatively, we could properly cfg() all tests here, but then we should have a way to enforce that no #[test] end up being generated for compiler builtins.

In the long term, I would prefer these tests to move to the libm-test crate, as proposed in #198 , since they are going to require more tools to actually work (the _eq functions at the top level of the libm crate do not cut it).

[gnzlbg]

The last commit moves the musl reference tests and required functionality into its own file.

[alexcrichton]

It may be worthwhile to point out that #[test] implicitly expands to #[cfg(test)]. There is no need to use both #[cfg(test)] and #[test] on a function.

I continue to not think that this change is worth it since it's more readable to have tests inline and I don't think there's a reason to move them out.