TST: Series construction from ExtensionDtype scalar
- [x] closes #28401
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

this looks fine.  should there be an analogous test in tests.extension? > this looks fine. should there be an analogous test in tests.extension?

Hmmm do you mean something like this?
``` python
In [7]: pd.array(pd.Interval(0, 1))
(...)
ValueError: Cannot pass scalar '(0, 1]' to 'pandas.array'.
```
We test this here:
https://github.com/pandas-dev/pandas/blob/52a1725965cb469de618ad56af467dc0aba53a87/pandas/tests/arrays/test_array.py#L287-L290
and 
https://github.com/pandas-dev/pandas/blob/52a1725965cb469de618ad56af467dc0aba53a87/pandas/tests/arrays/floating/test_construction.py#L85-L108
 thanks @arw2019 

FYI there maybe other places where we can use this fixture, if its simple, would take a PR, if not, pls open an issue. > FYI there maybe other places where we can use this fixture, if its simple, would take a PR, if not, pls open an issue.

Working on it. Probably will be a single PR but will see