Allow multiple parked threads in tests (#42551)

Closes #ISSUE

Release Notes:

- N/A

Co-Authored-By: Piotr <piotr@zed.dev>

crates/gpui/src/executor.rs

@@ -327,10 +327,8 @@ impl BackgroundExecutor {
                                 "parked with nothing left to run{waiting_message}{backtrace_message}",
                             )
                         }
-                        dispatcher.set_unparker(unparker.clone());
-                        parker.park_timeout(
-                            test_should_end_by.saturating_duration_since(Instant::now()),
-                        );
+                        dispatcher.push_unparker(unparker.clone());
+                        parker.park_timeout(Duration::from_millis(1));
                         if Instant::now() > test_should_end_by {
                             panic!("test timed out after {duration:?} with allow_parking")
                         }

crates/gpui/src/platform/test/dispatcher.rs

@@ -38,7 +38,7 @@ struct TestDispatcherState {
     waiting_backtrace: Option<Backtrace>,
     deprioritized_task_labels: HashSet<TaskLabel>,
     block_on_ticks: RangeInclusive<usize>,
-    last_parked: Option<Unparker>,
+    unparkers: Vec<Unparker>,
 }
 
 impl TestDispatcher {
@@ -58,7 +58,7 @@ impl TestDispatcher {
             waiting_backtrace: None,
             deprioritized_task_labels: Default::default(),
             block_on_ticks: 0..=1000,
-            last_parked: None,
+            unparkers: Default::default(),
         };
 
         TestDispatcher {
@@ -245,20 +245,14 @@ impl TestDispatcher {
         let block_on_ticks = lock.block_on_ticks.clone();
         lock.random.random_range(block_on_ticks)
     }
-    pub fn unpark_last(&self) {
-        self.state
-            .lock()
-            .last_parked
-            .take()
-            .as_ref()
-            .map(Unparker::unpark);
+
+    pub fn unpark_all(&self) {
+        self.state.lock().unparkers.retain(|parker| parker.unpark());
     }
 
-    pub fn set_unparker(&self, unparker: Unparker) {
-        let last = { self.state.lock().last_parked.replace(unparker) };
-        if let Some(last) = last {
-            last.unpark();
-        }
+    pub fn push_unparker(&self, unparker: Unparker) {
+        let mut state = self.state.lock();
+        state.unparkers.push(unparker);
     }
 }
 
@@ -299,7 +293,7 @@ impl PlatformDispatcher for TestDispatcher {
                 state.background.push(runnable);
             }
         }
-        self.unpark_last();
+        self.unpark_all();
     }
 
     fn dispatch_on_main_thread(&self, runnable: RunnableVariant) {
@@ -309,7 +303,7 @@ impl PlatformDispatcher for TestDispatcher {
             .entry(self.id)
             .or_default()
             .push_back(runnable);
-        self.unpark_last();
+        self.unpark_all();
     }
 
     fn dispatch_after(&self, duration: std::time::Duration, runnable: RunnableVariant) {