tests: add a critical flush() to run-tests.py to make output stable on py3

Differential Revision: https://phab.mercurial-scm.org/D5204

tests/run-tests.py

@@ -482,6 +482,7 b' def parseargs(args, parser):'
             parser.error('--with-hg must specify an executable hg script')
         if os.path.basename(options.with_hg) not in [b'hg', b'hg.exe']:
             sys.stderr.write('warning: --with-hg should specify an hg script\n')
+            sys.stderr.flush()
     if options.local:
         testdir = os.path.dirname(_bytespath(canonpath(sys.argv[0])))
         reporootdir = os.path.dirname(testdir)