Support analyzing test code

[zhassan-aws]

Use case: Unit tests in Rust are typically placed in a module that is guarded by #[cfg(test)], and each test is annotated with #[test]. The rmc and cargo rmc commands currently exclude code under #[cfg(test)] as well as functions annotated with #[test], so there is no way to analyze such code with RMC.

Requested feature: A new option to cargo rmc (e.g. --tests) that allows it to process test code under #[cfg(test)] and functions with the #[test] attribute. This would be similar in spirit to clippy's --tests option (rust-lang/rust-clippy#1436).

Link to relevant documentation (Rust reference, Nomicon, RFC):
https://doc.rust-lang.org/book/ch11-03-test-organization.html#the-tests-module-and-cfgtest
https://doc.rust-lang.org/reference/attributes/testing.html#the-test-attribute

Is this a breaking change? No