Add ServeSession test for project with folder

src/snapshots/serve_session__project_with_folder.snap

@@ -0,0 +1,30 @@
+---
+source: src/serve_session.rs
+expression: "view_tree(&session.tree(), &mut rm)"
+---
+id: id-1
+name: HelloWorld
+class_name: Folder
+properties: {}
+metadata:
+  ignore_unknown_instances: false
+  instigating_source:
+    Path: /foo/default.project.json
+  relevant_paths:
+    - /foo/src
+    - /foo/default.project.json
+children:
+  - id: id-2
+    name: hello
+    class_name: StringValue
+    properties:
+      Value:
+        Type: String
+        Value: "Hello, world!"
+    metadata:
+      ignore_unknown_instances: false
+      instigating_source:
+        Path: /foo/src/hello.txt
+      relevant_paths:
+        - /foo/src/hello.txt
+    children: []