in tests, change pd.arrays.SparseArray to SparseArray
- [x] closes https://github.com/pandas-dev/pandas/pull/30656#discussion_r363060184
- [x] tests added / passed
    - modified most tests that use `pd.arrays.SparseArray` to just import `SparseArray`
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
    - N/A

For @jreback to review based on his comment previous PR #30656 

In a few files (see below), left it as is because usage was pretty local (and allows `pd.arrays.SparseArray` reference to be tested in code)
```bash
$ grep -c -r arrays.SparseArray . | grep -v ":0"
./dtypes/test_generic.py:1
./frame/methods/test_quantile.py:2
./series/test_missing.py:2
```


