Add a TAP::Harness-compatible wrapper for usage by Test::Harness

[Leont]

If a user sets the HARNESS_SUBCLASS environmental variable, Test::Harness will use that class instead of using TAP::Harness. This can be used to leverage Test2::Harness in install tools like Extutils::MakeMaker, Module::Build and Module::Build::Tiny.

Writing a wrapper that has a Test::Harness-compatible-enough interface is probably fairly trivial, especially if one doesn't support every option.