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_composer/src/instrumentation.rs

@@ -23,9 +23,18 @@ static HARMONY_COMPOSER_EVENT_BUS: Lazy<broadcast::Sender<HarmonyComposerEvent>>
     });
 
 pub fn instrument(event: HarmonyComposerEvent) -> Result<(), &'static str> {
-    match HARMONY_COMPOSER_EVENT_BUS.send(event) {
-        Ok(_) => Ok(()),
-        Err(_) => Err("send error: no subscribers"),
+    #[cfg(not(test))]
+    {
+        match HARMONY_COMPOSER_EVENT_BUS.send(event) {
+            Ok(_) => Ok(()),
+            Err(_) => Err("send error: no subscribers"),
+        }
+    }
+
+    #[cfg(test)]
+    {
+        let _ = event; // Suppress the "unused variable" warning for `event`
+        Ok(())
     }
 }