TST: verify numpy.ravel works on a list of DataFrames with specified column names (#26247)
- [x] closes  #26247
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Inserted test to verify that np.ravel can be used on DataFrames with specified column names.
Hello @usersblock! 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:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2021-07-11 05:18:47 UTC Thanks for the pr @usersblock. Some comments, plus test you added is failing @usersblock please don't use the CI to try to get tests passing - see https://pandas.pydata.org/docs/development/ for how to run tests locally to make sure they pass @jreback @mzeitlin11 
My bad for doing that. There was one part of the workflow I might not understand. 
```
# this works - list of DataFrames without column names
np.ravel([pd.DataFrame(batch.reshape(1,3)) for batch in x])

# this also works - single DataFrame with column names
np.ravel(pd.DataFrame(x[0].reshape(1,3), columns=["x1", "x2", "x3"]))

# this doesn't work - list of DataFrames with column names
np.ravel([pd.DataFrame(batch.reshape(1,3), columns=["x1", "x2", "x3"]) for batch in x])
```
From the following code from the issue, the third np.ravel fails py38_macos and py38_np18. It passes on my local environment, py39 and py38_32bit. Could this be a compatibility issue? I'd check the numpy versions on each of the runs - that seems like the most likely candidate for a difference like this Numpy 1.18.5 is the culprit for the failing tests. Perhaps the original issue should be reclassified as a bug fix for compatibility issues.