When I try to install SimpleTest via Extend > Testing > Install, I get the following error:
The testing framework requires the sites/simpletest directory to exist and be writable in order to run tests. (Currently using Simpletest site directory version Missing)
I have run
$ sudo chmod -R 777 sites/simpletest in an attempt at mediating this issue, but to no avail.
I am running Drupal 8.3.5 on Fedora 25.