don't crash on command lists containing non-strings

slave/buildslave/test/fake/runprocess.py

@@ -101,7 +101,7 @@ def __init__(self, builder, command, workdir, **kwargs):
                     msg.append('%s: expected %r,\n  got %r' % (key, exp.kwargs[key], kwargs[key]))
             if msg:
                 msg.insert(0, 'did not get expected __init__ arguments for\n '
-                              + " ".join(kwargs.get('command', ['unknown command'])))
+                              + " ".join(map(str, kwargs.get('command', ['unknown command']))))
                 self._expectations[:] = [] # don't expect any more instances, since we're failing
                 raise AssertionError("\n".join(msg))