tests: force a stable hostname in patchbomb tests
No visible output changes in this commit because everything is globbed
away, but on Python 3 the stable hostname will mean that the headers
don't trigger RFC2822 multi-line mode because they'll always be
consistently short.
Differential Revision: https://phab.mercurial-scm.org/D4295
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}