Tests that show register shutdown function issue (#70) is invalid

[justinmaurerdotdev]

This addresses https://github.com/wp-cli/cron-command/issues/70. This one is sort of complicated to reproduce accurately, but these tests demonstrate that a registered shutdown function is capable of writing to a file, so the reported issue is invalid, and can probably be safely closed as a "will not fix". Brian and I decided that we preferred the error_log() demonstration over file_put_contents() for Behat, but we tested both ways and the result was the same.

Submitted during Contributor Day.