tools: open `test.tap` file in write-binary mode

By default the logfile is opened in append mode. This commit makes sure
that the file is opened in write-binary mode, so that the file will be
created if it doesn't exist or overwrite if it exists.

Fixes: #2834

PR-URL: #2837
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Alexis Campailla <orangemocha@nodejs.org>