BUG: test_contains checks wrong non-missing element
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- [x] I have confirmed this bug exists on the latest version of pandas.

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

---

#### Problem description

The `test_contains` test for `ExtensionArray` objects has the following lines:

```python
# first elements are non-missing
assert data[0] in data
assert data_missing[0] in data_missing
```

However the first element in `data_missing` IS missing, and this test gives an error if `__getitem__` returns the physical representation of the NA object instead of `pd.NA`, as this needs not to return `True` for `__contains__` (unless I am missing something). Either way, the comment makes clear that this line intended to access a non NA object.

#### Expected Output

The lines should be changed to:

```python
# first and second elements are respectively non-missing
assert data[0] in data
assert data_missing[1] in data_missing
```