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

betterproto/tests/inputs/mapmessage/mapmessage.json

@@ -0,0 +1,10 @@
+{
+  "items": {
+    "foo": {
+      "count": 1
+    },
+    "bar": {
+      "count": 2
+    }
+  }
+}

betterproto/tests/inputs/mapmessage/mapmessage.proto

@@ -0,0 +1,9 @@
+syntax = "proto3";
+
+message Test {
+  map<string, Nested> items = 1;
+}
+
+message Nested {
+  int32 count = 1;
+}