Task: Integration test for the protoc-gen bin

[arthurnn]

Do we test the protocol-gen-twirp_ruby ?

1. I see the go bin has a main_test.go file, but I don't see we call it during our action CI build
   Note: cd protoc-gen-twirp_ruby && go test ./... will execute the test
2. We could have a integration test, where from a .proto file we generate the stubs, and asserts against that, to prevent major breaking changes.
3. Idea: bring some of the example app as part of our test suite.

1 is a easy win, 2 and 3 are more like ideas, but the overall goal here is to have tests that trigger the protoc ruby bin.