feat: replace event channel with broadcast channel

This makes `EventTracker` receive events immediately
instead of being moved from event emitter to event tracker
by a task spawned from `TestContext::new_internal`.

This makes `EventTracker.clear_events` reliable
as it is guaranteed to remove all events emitted
by the time it is called rather than only events
that have been moved already.

Cargo.lock

@@ -227,6 +227,18 @@ dependencies = [
  "syn 1.0.109",
 ]
 
+[[package]]
+name = "async-broadcast"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "258b52a1aa741b9f09783b2d86cf0aeeb617bbf847f6933340a39644227acbdb"
+dependencies = [
+ "event-listener 5.2.0",
+ "event-listener-strategy",
+ "futures-core",
+ "pin-project-lite",
+]
+
 [[package]]
 name = "async-channel"
 version = "1.9.0"
@@ -1149,6 +1161,7 @@ version = "1.137.3"
 dependencies = [
  "ansi_term",
  "anyhow",
+ "async-broadcast",
  "async-channel 2.2.0",
  "async-imap",
  "async-native-tls",