Organize test-cases into folders, extract compatibility test into proper test, support adding test-case specific tests

betterproto/tests/inputs/nestedtwice/nestedtwice.json

@@ -0,0 +1,11 @@
+{
+  "root": {
+    "name": "double-nested",
+    "parent": {
+      "child": [{"foo": "hello"}],
+      "enumChild": ["A"],
+      "rootParentChild": [{"a": "hello"}],
+      "bar": true
+    }
+  }
+}