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

tests/run-tests.py

@@ -1068,6 +1068,7 b' class Test(unittest.TestCase):'
         env["HGUSER"]   = "test"
         env["HGENCODING"] = "ascii"
         env["HGENCODINGMODE"] = "strict"
+        env["HGHOSTNAME"] = "test-hostname"
         env['HGIPV6'] = str(int(self._useipv6))
 
         extraextensions = []