Fix flaky unit tests due to non-existent cache

tests/all/utils/fixture.rs

@@ -322,7 +322,9 @@ impl Fixture {
     }
 
     pub fn cache(&self) -> Cache {
-        Cache::at(&self.cache_dir())
+        let cache_dir = self.cache_dir();
+        fs::create_dir_all(&cache_dir).unwrap();
+        Cache::at(&cache_dir)
     }
 
     /// The `step_install_wasm_bindgen` and `step_run_wasm_bindgen` steps only