Add the ability to unit test the output of protoc-gen-kotlin

[garyp]

Create some test scaffolding to allow testing that the source code produced by protoc-gen-kotlin is compilable by the Kotlin compiler.

Probably the unit test would take a CodeGeneratorRequest as input, run it through pbandk.gen.runGenerator(), and use a library like https://github.com/tschuchortdev/kotlin-compile-testing for running the Kotlin compiler on the CodeGeneratorResponse output. Even better would be to have the unit test take the source of a .proto file as input and the test would run protoc in order to generate the CodeGeneratorRequest.