tests: Use more default settings
The download URL defaults to the public download URL and the version of pressure-vessel defaults to the latest standalone release, so we don't usually need to specify those two explicitly.
In the case of PRESSURE_VESSEL_DOWNLOAD_URL, if it has been explicitly set, we still want to pass it in: the official CI does this to bypass the CDN that implements repo.steampowered.com and go directly to the origin server.
/cc @denittis