BUG: pd.testing.makeMultiIndex ignores k parameter
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- [x] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

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#### Code Sample, a copy-pastable example

```python
assert len(pd.testing.makeMultiIndex(k=10)) == 4
```

https://github.com/pandas-dev/pandas/blob/0976c4ca0276a333346619c5a9e3c92631ec7006/pandas/_testing/__init__.py#L491-L492

#### Problem description

All other `testing.make*Index` methods take a parameter `k` which lets you specify the length. `makeMultiIndex` explicitly takes this parameter but does nothing with it.

#### Expected Output

```python
assert len(pd.testing.makeMultiIndex(k=10)) == 10
```
