Support test output in TAP format

Hello.

I have been working with Heroku CI for Ruby on Rails for a few days and have had a hard time finding any support for outputting test results in the TAP-format (https://testanything.org/tap-specification.html).

I have now created my own extended minitest_reporter for our usecase but I wondered if there would be a possibility to support it in the buildpack itself.

**My questions**

- Would it make sense to add TAP-output for minitest and rspec?
- What cases would you need to solve and how should it be configured?
- What makes this hard to add?

With some guidance I might be able to create a draft for an implementation.

The issue names Heroku CI, the Ruby buildpack, minitest, rspec, and a custom minitest_reporter, but no file or test. Start by locating the buildpack’s existing test-runner handling and comparing it with that reporter; done means an agreed configuration and working TAP output for the requested runners.

Written by the indexing model from the issue text.