Support checking for Rust test functions.

Without the `--test` flag, functions marked as `#[test]` are not checked for errors. Previously a file like this:

```rust
pub fn totally_fine() {
  println!("hey!");
}

#[test]
fn test_oops() {
  eirjgirogoregjrejgierjgorjgoerjgo // <-- error
}
```

would appear to be correct to Flycheck.