TAP output support

[ELLIOTTCABLE]

I'd really love a command-line flag to the test-runner, analogous to -verbose, that allows me to generate TAP-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 or similar, command-line integration tests using BATS, 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

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 really cool reporters, y'dig?)

   

[ghost]

This would require some cooperation from the build system as well, so dune is probably the right place to open a master ticket to track these ideas