ci: have rust-cargo-test inherit from all
This help changing configuration for everything at the same time.
This was initially the case before being dropped by mistake in 0ddd5e1f5f67.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}