…esting + improve how ENV_FILE works (#8116)

Our react-scripts setup is different in that our client is in a
subfolder, and this was not taken into consideration when resolving
files that reside in the root folder, not in the client folder.

Futhermore, the client would also load other environment files.
To be consistent with the rest of yari, we now only load `.env`,
unless `ENV_FILE` is set, in which case we load that file instead.

Finally, the static-server is no longer run with the testing env file.