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> {