don't fail with "expected instances not created" on wrong instance

slave/buildslave/test/fake/runprocess.py

@@ -101,6 +101,7 @@ def __init__(self, builder, command, workdir, **kwargs):
                     msg.append('%s: expected %r, got %r' % (key, exp.kwargs[key], kwargs[key]))
             if msg:
                 msg.insert(0, 'did not get expected __init__ arguments:')
+                self._expectations[:] = [] # don't expect any more instances, since we're failing
                 raise AssertionError("\n".join(msg))
 
         self._builder = builder