Add support for serving files within a test directory through echoecho

In order to test valid serving of files in combination with things like:
* get
* post
* delays

Yeti needs to specify the new `dirroot` argument to echoecho so that files served. This was added in 0.1.9 of echoecho.

You can then use echoecho to retrieve URLs like `echo/delay/3/get?file=assets/checknet.txt` which are converted into URLs such as `tests/unit/moodle-core-checknet/main/echo/delay/3/get?file=assets/checknet.txt` by Yeti, and `tests/unit/moodle-core-checknet/main/assets/checknet.txt` by EchoEcho.