digital_io: add clippy exception for the #[cfg(test)] variant of mod gpio

Clippy disallows #[cfg(test)] mods that are followed by other code by
default[1] and usually this makes sense, because tests should be at the
end of the file, but in our case the mod does not contain tests but instead
imports a test-specific variant of the gpio mod.

Allow that.

[1]: https://rust-lang.github.io/rust-clippy/master/index.html#/items_after_test_module

Signed-off-by: Leonard Göhrs <l.goehrs@pengutronix.de>

Author: hnez

src/digital_io.rs

@@ -22,6 +22,7 @@ use async_std::task::spawn;
 use crate::broker::{BrokerBuilder, Topic};
 use crate::led::BlinkPattern;
 
+#[allow(clippy::items_after_test_module)]
 #[cfg(test)]
 mod gpio {
     mod test;