TST: split up test_concat.py #37243
- [ ] closes #37243 
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @laffite56! Thanks for updating 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/reshape/concat/test_empty.py`](https://github.com/pandas-dev/pandas/blob/dcb2717e2cfaa80a34e057540bc644019df42259/pandas/tests/reshape/concat/test_empty.py):

> [Line 84:89](https://github.com/pandas-dev/pandas/blob/dcb2717e2cfaa80a34e057540bc644019df42259/pandas/tests/reshape/concat/test_empty.py#L84): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (91 > 88 characters)



* In the file [`pandas/tests/reshape/concat/test_series.py`](https://github.com/pandas-dev/pandas/blob/dcb2717e2cfaa80a34e057540bc644019df42259/pandas/tests/reshape/concat/test_series.py):

> [Line 230:89](https://github.com/pandas-dev/pandas/blob/dcb2717e2cfaa80a34e057540bc644019df42259/pandas/tests/reshape/concat/test_series.py#L230): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (89 > 88 characters)





##### Comment last updated at 2020-10-20 17:03:34 UTC I have put a few functions into classes, but i don't know if this is fine.  > I have put a few functions into classes, but i don't know if this is fine.

I generally would go the other way actually.

But most important for this PR. please let's do just straight copy-paste, its very hard to see changes otherwise. my requests were all about just moving existing code around in blocks, not actually change that much.

so we want to merge this soon. I am also happy to do this in a separate PR by first literally moving the entire file, then moving pieces of it.
 I'm a little confused. You want me to create a new PR, with one commit containing only copy-paste original "test_concat.py" to new folder, like i have right now:

from **pandas/tests/reshape/test_concat.py**  => **pandas/tests/reshape/concat/test_concat.py**

and then in next commit reshape "test_concat.py" to submodules?
 Sir, @jreback, i don't really know what to do with this issue. I can easly make new PR with every commit separated, like one commit for moving one file, then next ones for every new submodule and for every significant change, so You'll be able to see every change.  > Sir, @jreback, i don't really know what to do with this issue. I can easly make new PR with every commit separated, like one commit for moving one file, then next ones for every new submodule and for every significant change, so You'll be able to see every change.

so 1) pls merge master. I would actually just do a straight move & split and NOTHING else in a the first pr. split by classes is fine.
let's do that and will merge quickly. Then its very easy to do followups. #37360 here is new PR.