TST: corrwith and tshift in groupby/groupby.transform
This PR tests `corrwith` and `tshift`, so not sure if it fully closes the original issue or not, but the other functions`backfill`, `pad` and `cumcount` don't seem to be supported in `groupby.transform` as reported in issues  #27472 and #31269.

- [x] closes #27905 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`


@ryankarlos is there more to add. if not I don't think this closes the issue? @simonjayhawkins  yes Im thinking of  adding for `tshift` and `corrwith` - anymore ? > @simonjayhawkins yes Im thinking of adding for `tshift` and `corrwith` 

those are explicitly mentioned in the issue, so could add here or seperate PRs and change the OP of this PR to xref instead of closes.

> - anymore ?

not sure. @charlesdong1991 ?

 So there is already a fixture called `transformation_func` which has all of these; I think should be using that in tests where there is a gap today instead of adding this Thanlks @WillAyd , Ive now added new tests with the `transformation_func` fixture. @jreback  @WillAyd  thanks for the comments - addressed them now. 
 > @simonjayhawkins yes Im thinking of adding for `tshift` and `corrwith` - anymore ?

looking at 

https://github.com/pandas-dev/pandas/blob/53ece7009c02eefc0befccf911d72369b0ed37f8/pandas/tests/groupby/test_transform.py#L330-L332

it appears that pad and cumcount need to be tested also. > > @simonjayhawkins yes Im thinking of adding for `tshift` and `corrwith` - anymore ?
> 
> looking at
> 
> https://github.com/pandas-dev/pandas/blob/53ece7009c02eefc0befccf911d72369b0ed37f8/pandas/tests/groupby/test_transform.py#L330-L332
> 
> it appears that pad and cumcount need to be tested also.

So these other functions were throwing `AttributeError` when used with transform which are also mentioned in  issues #31269 and #27472   @simonjayhawkins  to be honest, think this original issue may be out of date and superseded by #32293 ,  #31269 , #27472 and #32344 reporting the associated bugs which need to be fixed first for `cumcount`, `pad`, `backfill`, `tshift`, `corrwith`, as seems rest of transform funcs are already tested.

If you want I could just leave the tests for `groupby` and put them in `test_groupby.py` - otherwise feel free to close this PR if not relevant anymore ? > otherwise feel free to close this PR if not relevant anymore ?

Thanks @ryankarlos for the work here, much appreciated, but I'll close this PR and close the issue too.