manifestcache: clear the cache before testing the debug command
Right now the cache is empty before this section of the test. However, we are
about to improve the persistence of the cache (putting it under `wllock`,
intead of `lock`). So we install the cleanup before.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}