run-tests: error out on `--local --with-[c]hg`

I don't see much reason to allow these combinations. You could use
--local and override only one of --with-hg or --with-chg, but I don't
see much practical use for that. It would be easy to work around
anyway by passing both --with-hg and --with-chg. By erroring out, it
makes the code a bit easier to reason about to allow the next few
patches.

Differential Revision: https://phab.mercurial-scm.org/D6759

tests/run-tests.py

@@ -493,6 +493,8 b' def parseargs(args, parser):'
             sys.stderr.write('warning: --with-hg should specify an hg script\n')
             sys.stderr.flush()
     if options.local:
+        if options.with_hg or options.with_chg:
+            parser.error('--local cannot be used with --with-hg or --with-chg')
         testdir = os.path.dirname(_bytespath(canonpath(sys.argv[0])))
         reporootdir = os.path.dirname(testdir)
         pathandattrs = [(b'hg', 'with_hg')]