Tests use a lot of duplicate points

Since we added the warning about re-evaluating the objective at the same point our tests are full of warnings:
```
scikit-optimize/skopt/optimizer/optimizer.py:174: UserWarning: The objective has been evaluated at this point before.
  warnings.warn("The objective has been evaluated at this point before.")
```

We should think whether we want t mute the warning or improve the tests.