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

betterproto/tests/inputs/nested/nested.json

@@ -0,0 +1,6 @@
+{
+  "nested": {
+    "count": 150
+  },
+  "sibling": {}
+}

betterproto/tests/inputs/nested/nested.proto

@@ -0,0 +1,18 @@
+syntax = "proto3";
+
+// A test message with a nested message inside of it.
+message Test {
+    // This is the nested type.
+    message Nested {
+      // Stores a simple counter.
+      int32 count = 1;
+    }
+
+    Nested nested = 1;
+    Sibling sibling = 2;
+    Sibling sibling2 = 3;
+}
+
+message Sibling {
+  int32 foo = 1;
+}