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
patchbomb: allow using HGHOSTNAME to force a hostname
I'll update run-tests.py to set this globally to stabilize some
tests. The variable name is intentionally generic because I suspect we
should generalize this to other tests.
Differential Revision: https://phab.mercurial-scm.org/D4294