Bring back zed.rs tests (#3907)

At present 3 tests still fail; 2 are related to keymap issues that (I
believe) @maxbrunsfeld is working on. The other one
(`test_open_paths_action`) I'll look into. edit: done
This PR also fixes workspace unregistration, as we've put the code to do
that behind `debug_assert`
(https://github.com/zed-industries/zed/pull/3907/files#diff-041673bbd1947a35d45945636c0055429dfc8b5985faf93f8a8a960c9ad31e28L649).
Release Notes:
- N/A

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

@@ -266,7 +266,7 @@ impl Platform for TestPlatform {
     }
 
     fn local_timezone(&self) -> time::UtcOffset {
-        unimplemented!()
+        time::UtcOffset::UTC
     }
 
     fn path_for_auxiliary_executable(&self, _name: &str) -> Result<std::path::PathBuf> {

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

@@ -18,7 +18,7 @@ pub struct TestWindowState {
     pub(crate) edited: bool,
     platform: Weak<TestPlatform>,
     sprite_atlas: Arc<dyn PlatformAtlas>,
-
+    pub(crate) should_close_handler: Option<Box<dyn FnMut() -> bool>>,
     input_callback: Option<Box<dyn FnMut(InputEvent) -> bool>>,
     active_status_change_callback: Option<Box<dyn FnMut(bool)>>,
     resize_callback: Option<Box<dyn FnMut(Size<Pixels>, f32)>>,
@@ -44,7 +44,7 @@ impl TestWindow {
             sprite_atlas: Arc::new(TestAtlas::new()),
             title: Default::default(),
             edited: false,
-
+            should_close_handler: None,
             input_callback: None,
             active_status_change_callback: None,
             resize_callback: None,
@@ -117,6 +117,9 @@ impl TestWindow {
 
         self.0.lock().input_handler = Some(input_handler);
     }
+    pub fn edited(&self) -> bool {
+        self.0.lock().edited
+    }
 }
 
 impl PlatformWindow for TestWindow {
@@ -235,8 +238,8 @@ impl PlatformWindow for TestWindow {
         self.0.lock().moved_callback = Some(callback)
     }
 
-    fn on_should_close(&self, _callback: Box<dyn FnMut() -> bool>) {
-        unimplemented!()
+    fn on_should_close(&self, callback: Box<dyn FnMut() -> bool>) {
+        self.0.lock().should_close_handler = Some(callback);
     }
 
     fn on_close(&self, _callback: Box<dyn FnOnce()>) {