.arcconfig

run-tests: use symbolic constant instead of arbitrary number line matching

(This is a gratuitous cleanup that I made while investigating a bug).

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}