Improve getLink method for unit tests

Summary:
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.

Test Plan: This diff.

Reviewers: avivey, Blessed Reviewers, epriestley

Reviewed By: avivey, Blessed Reviewers, epriestley

Subscribers: aurelijus, Korvin, epriestley

Maniphest Tasks: T7977

Differential Revision: https://secure.phabricator.com/D12664