Added diff testing file
Should test the diff for a specific series.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @Unprocessable! Thanks for opening this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:

* In the file [`pandas/tests/series/test_diff.py`](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py):

> [Line 7:1](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L7): [E302](https://duckduckgo.com/?q=pep8%20E302) expected 2 blank lines, found 1
> [Line 8:21](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L8): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 8:24](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L8): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 8:27](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L8): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 8:30](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L8): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 8:33](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L8): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 8:36](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L8): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 8:39](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L8): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 8:42](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L8): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 8:44](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L8): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 8:47](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L8): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 8:50](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L8): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 8:52](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L8): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 8:55](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L8): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 8:58](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L8): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 8:61](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L8): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 8:64](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L8): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 10:31](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L10): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 10:44](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L10): [E251](https://duckduckgo.com/?q=pep8%20E251) unexpected spaces around keyword / parameter equals
> [Line 10:46](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L10): [E251](https://duckduckgo.com/?q=pep8%20E251) unexpected spaces around keyword / parameter equals
> [Line 12:89](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L12): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (144 > 88 characters)
> [Line 16:89](https://github.com/pandas-dev/pandas/blob/2c8e53efa2f5c6470a9d65da55461f3d3ea9b961/pandas/tests/series/test_diff.py#L16): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (121 > 88 characters)

