Rename deprioritize_task -> deprioritize

It applies to a family of tasks, not a task.

crates/gpui2/src/app/test_context.rs

@@ -354,6 +354,7 @@ impl<T: Send> Model<T> {
             })
         });
 
+        // Run other tasks until the event is emitted.
         loop {
             match rx.try_next() {
                 Ok(Some(event)) => return event,

crates/gpui2/src/executor.rs

@@ -245,7 +245,7 @@ impl BackgroundExecutor {
     }
 
     #[cfg(any(test, feature = "test-support"))]
-    pub fn deprioritize_task(&self, task_label: TaskLabel) {
+    pub fn deprioritize(&self, task_label: TaskLabel) {
         self.dispatcher.as_test().unwrap().deprioritize(task_label)
     }