The official documentation in Change test runner via the command line
provides an example of running test/run_test.rb with --runner tap, but this results in an error:

$ ruby test/run_test.rb --runner tap
invalid argument: --runner tap
(snip)

However, checking the available options show --runner only accepts console, emacs, xml:

$ ruby test/run.rb --help | grep -A1 -- --runner=
    -r, --runner=RUNNER              Use the given RUNNER.
                                     (c[onsole], e[macs], x[ml])

If tap should be supported, update the implementation to allow it.
If tap is not supported, update the documentation to console, emacs or xml.

Could you take a look?