Enable clippy::option_map_unit_fn (#8751)

This PR enables the
[`clippy::option_map_unit_fn`](https://rust-lang.github.io/rust-clippy/master/index.html#/option_map_unit_fn)
rule and fixes the outstanding violations.

Release Notes:

- N/A

crates/gpui/src/test.rs

@@ -72,7 +72,9 @@ pub fn run_test(
                         if is_randomized {
                             eprintln!("failing seed: {}", seed);
                         }
-                        on_fail_fn.map(|f| f());
+                        if let Some(f) = on_fail_fn {
+                            f()
+                        }
                         panic::resume_unwind(error);
                     }
                 }