Ref T7977. The PhutilTestCase::getLink method currently relies on arcanist projects instead of repositories. Instead, make this logic a bit smarter by looking up the base URI from phabricator.uri (currently it is hardcoded to https://secure.phabricator.com).
Ideally, we would pass ?repositories=$REPOSITORY_PHID to DiffusionSymbolController as well, but I don't know if this is worth pursuing.