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?)

   

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