Cleanup tests/scalar, isolate scalar-only tests from scalar-mixed tests
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/scalar/nat/test_nat.py`](https://github.com/jbrockmendel/pandas/blob/5719b6390e09df874987050d715d6a45bdc25c81/pandas/tests/scalar/nat/test_nat.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/nat/test_vector_compat.py`](https://github.com/jbrockmendel/pandas/blob/5719b6390e09df874987050d715d6a45bdc25c81/pandas/tests/scalar/nat/test_vector_compat.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/period/test_asfreq.py`](https://github.com/jbrockmendel/pandas/blob/5719b6390e09df874987050d715d6a45bdc25c81/pandas/tests/scalar/period/test_asfreq.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/period/test_period.py`](https://github.com/jbrockmendel/pandas/blob/5719b6390e09df874987050d715d6a45bdc25c81/pandas/tests/scalar/period/test_period.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/period/test_vector_compat.py`](https://github.com/jbrockmendel/pandas/blob/5719b6390e09df874987050d715d6a45bdc25c81/pandas/tests/scalar/period/test_vector_compat.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/timedelta/test_arithmetic.py`](https://github.com/jbrockmendel/pandas/blob/5719b6390e09df874987050d715d6a45bdc25c81/pandas/tests/scalar/timedelta/test_arithmetic.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/timedelta/test_construction.py`](https://github.com/jbrockmendel/pandas/blob/5719b6390e09df874987050d715d6a45bdc25c81/pandas/tests/scalar/timedelta/test_construction.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/timedelta/test_timedelta.py`](https://github.com/jbrockmendel/pandas/blob/5719b6390e09df874987050d715d6a45bdc25c81/pandas/tests/scalar/timedelta/test_timedelta.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/timedelta/test_vector_compat.py`](https://github.com/jbrockmendel/pandas/blob/5719b6390e09df874987050d715d6a45bdc25c81/pandas/tests/scalar/timedelta/test_vector_compat.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/timestamp/test_arithmetic.py`](https://github.com/jbrockmendel/pandas/blob/5719b6390e09df874987050d715d6a45bdc25c81/pandas/tests/scalar/timestamp/test_arithmetic.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/timestamp/test_rendering.py`](https://github.com/jbrockmendel/pandas/blob/5719b6390e09df874987050d715d6a45bdc25c81/pandas/tests/scalar/timestamp/test_rendering.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/timestamp/test_timestamp.py`](https://github.com/jbrockmendel/pandas/blob/5719b6390e09df874987050d715d6a45bdc25c81/pandas/tests/scalar/timestamp/test_timestamp.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/timestamp/test_timezones.py`](https://github.com/jbrockmendel/pandas/blob/5719b6390e09df874987050d715d6a45bdc25c81/pandas/tests/scalar/timestamp/test_timezones.py) !

 Too much going on; closing for the time being.