Add basic oneof tests

betterproto/tests/oneof-name.json

@@ -0,0 +1,3 @@
+{
+  "name": "foo"
+}

betterproto/tests/oneof.json

@@ -0,0 +1,3 @@
+{
+  "count": 1
+}

betterproto/tests/oneof.proto

@@ -0,0 +1,8 @@
+syntax = "proto3";
+
+message Test {
+  oneof foo {
+    int32 count = 1;
+    string name = 2;
+  }
+}