fix: prevent instrumentation to run in test mode (#102)

The CI pipeline (`./check.sh`) was failing because of test errors, which was caused by the instrumentation framework complaining that no subscribers/listeners were registered.

Instead of setting up all tests to run with a dummy subscriber, move the implementation of the instrumentation behind a feature flag so that it runs only for tests.

There's a catch though: the `#[cfg(test)]` directive works only when directly testing the crate. If a crate `A` depends on another crate `B`, `B` will be compiled as usual (aka not in test mode) which will not trigger the `test` flag.

So we need to introduce our own `testing` feature flag for `harmony` core and import it with that flag (only during dev/test).

More info: https://github.com/rust-lang/rust/issues/59168

Co-authored-by: Ian Letourneau <letourneau.ian@gmail.com>
Reviewed-on: https://git.nationtech.io/NationTech/harmony/pulls/102

harmony_cli/src/lib.rs

@@ -163,7 +163,7 @@ async fn init<T: Topology + Send + Sync + 'static>(
 }
 
 #[cfg(test)]
-mod test {
+mod tests {
     use harmony::{
         inventory::Inventory,
         maestro::Maestro,