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

betterproto/tests/inputs/repeatedmessage/repeatedmessage.json

@@ -0,0 +1,10 @@
+{
+  "greetings": [
+    {
+      "greeting": "hello"
+    },
+    {
+      "greeting": "hi"
+    }
+  ]
+}

betterproto/tests/inputs/repeatedmessage/repeatedmessage.proto

@@ -0,0 +1,11 @@
+syntax = "proto3";
+
+package repeatedmessage;
+
+message Test {
+  repeated Sub greetings = 1;
+}
+
+message Sub {
+  string greeting = 1;
+}