TAP output support

I'd really love a command-line flag to the test-runner, analogous to `-verbose`, that allows me to generate [TAP](https://testanything.org/)-output from running tests instead of a custom format. This would allow:

1. Integration of multiple external testing systems upon `make test` (JavaScript-side tests [using Mocha](https://mochajs.org/#tap) or similar, command-line integration tests [using BATS](https://github.com/bats-core/bats-core), etceteras.)
2. Integration of multiple internal testing systems' output (should I open an identical Issue on ppx_expect?)
3. Integration with any higher-level [TAP consumers](http://testanything.org/consumers.html)
4. Beautification of test-output without the writing and maintenance of a ton of colourization and such code in this library itself (TAP is designed to be piped into [some](https://github.com/scottcorgan/tap-spec) really [cool](https://github.com/tapjs/tap-mocha-reporter) reporters, [y'dig](https://github.com/zoubin/tap-summary)?)


Start by locating the test-runner's command-line flag handling and the existing verbose/custom output path. Trace how test results are emitted, then define the TAP output entry point and verify that the runner can produce TAP suitable for the external consumers described in the issue.

Written by the indexing model from the issue text.