Exercise same interface used in file

In the ruby_install_layer we're exercising `cached_layer_write_metadata` but we were testing `restored_layer_action` which is called currently but might not be in the future via refactoring. This change asserts the behavior we want while using the exact same interface we are currently using.

buildpacks/ruby/src/layers/ruby_install_layer.rs

@@ -249,7 +249,7 @@ pub(crate) enum RubyInstallError {
 
 #[cfg(test)]
 mod tests {
-    use crate::layers::shared::restored_layer_action;
+    use crate::layers::shared::temp_build_context;
 
     use super::*;
 
@@ -325,30 +325,35 @@ version = "3.1.3"
 
     #[test]
     fn test_ruby_version_difference_clears_cache() {
+        let temp = tempfile::tempdir().unwrap();
+        let context = temp_build_context::<RubyBuildpack>(temp.path());
         let old = Metadata {
             ruby_version: ResolvedRubyVersion("2.7.2".to_string()),
             distro_name: "ubuntu".to_string(),
             distro_version: "20.04".to_string(),
             cpu_architecture: "x86_64".to_string(),
         };
         let differences = old.diff(&old);
-        let actual = restored_layer_action(&old, &old);
-        assert!(matches!(
-            actual,
-            (libcnb::layer::RestoredLayerAction::KeepLayer, _)
-        ));
-
         assert_eq!(differences, Vec::<String>::new());
 
+        cached_layer_write_metadata(layer_name!("ruby"), &context, &old).unwrap();
+        let result = cached_layer_write_metadata(layer_name!("ruby"), &context, &old).unwrap();
+        let actual = result.state;
+        assert!(matches!(actual, LayerState::Restored { .. }));
+
         let now = Metadata {
             ruby_version: ResolvedRubyVersion("3.0.0".to_string()),
             ..old.clone()
         };
+        let differences = now.diff(&old);
+        assert_eq!(differences.len(), 1);
 
-        let actual = restored_layer_action(&old, &now);
+        let result = cached_layer_write_metadata(layer_name!("ruby"), &context, &now).unwrap();
         assert!(matches!(
-            actual,
-            (libcnb::layer::RestoredLayerAction::DeleteLayer, _)
+            result.state,
+            LayerState::Empty {
+                cause: EmptyLayerCause::RestoredLayerAction { .. }
+            }
         ));
     }
 }