doc: note that tests should include a description

Update the Writing Tests guide to specify that tests should include a
brief description of what they are designed to test.

PR-URL: https://github.com/nodejs/node/pull/9415



Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com&gt;oc>