Remove Executor trait from collab and use an enum instead

This will let us save off the executor and avoid using generics.

crates/collab/src/executor.rs

@@ -0,0 +1,36 @@
+use std::{future::Future, time::Duration};
+
+#[derive(Clone)]
+pub enum Executor {
+    Production,
+    #[cfg(test)]
+    Deterministic(std::sync::Arc<gpui::executor::Background>),
+}
+
+impl Executor {
+    pub fn spawn_detached<F>(&self, future: F)
+    where
+        F: 'static + Send + Future<Output = ()>,
+    {
+        match self {
+            Executor::Production => {
+                tokio::spawn(future);
+            }
+            #[cfg(test)]
+            Executor::Deterministic(background) => {
+                background.spawn(future).detach();
+            }
+        }
+    }
+
+    pub fn sleep(&self, duration: Duration) -> impl Future<Output = ()> {
+        let this = self.clone();
+        async move {
+            match this {
+                Executor::Production => tokio::time::sleep(duration).await,
+                #[cfg(test)]
+                Executor::Deterministic(background) => background.timer(duration).await,
+            }
+        }
+    }
+}