TST: Add numeric coercion tests
- [x] related to #12747 and #12780
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`

Because dtype coercion affects to lots of functions, would like to fix in the following steps. The behavior which looks incorrect are commented with `TODO_GH12747`

1(this PR) Add numeric related tests to clarify the current behaviour (and future change) 
2. Fix numeric coercion.
3. Add datetime-like tests.
4. Fix datetime-like coercion.

@jreback OK, created `test_coercion.py`.
 thanks!
