caucase.test: Do not fail when decoding caucase.sh stdout

Print any ascii characters from its output, substituting those which are
not.

caucase/test.py

@@ -3892,7 +3892,7 @@ class CaucaseShellTest(TestCase):
       'Process %r exited with status %s, dumping output:\n%s' % (
         command,
         status,
-        stdout.decode('ascii'),
+        stdout.decode('ascii', errors='replace'),
       )
     )