Test Anything Protocol output for SRFI-64

[Test Anything Protocol](https://testanything.org/)


## Usage

    (import (scheme base)
            (srfi 64)
            (retropikzel tap))

    (test-runner-current (tap-runner))


## Documentation

(**tap-runner**)

Get the TAP test runner.

(**set-tap-runner-output-port!** port)

Set the output port of the test runner. To output to file you can do:

    (define tap-file "tap-result.txt")
    (when (file-exists? tap-file) (delete-file tap-file))
    (set-tap-runner-output-port! (open-output-file tap-file))


### Environment variables

**SCM_TAP_EXIT_FAIL**

If you want the test runner to exit on end with fail (1) if any of the tests
fail, set this environment variable.