TST/CLN: follow-up to #24750
cc @jreback 

`test_where_unsafe_itemsize_fail` changed to `test_where_upcast` in #24750

this follow-on PR merges `test_where_upcast` into `test_where_unsafe_upcast` to avoid duplication.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24810?src=pr&el=h1) Report
> Merging [#24810](https://codecov.io/gh/pandas-dev/pandas/pull/24810?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/17a6bc56e5ab6ad3dab12d3a8b20ed69a5830b6f?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24810/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24810?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24810   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files         166      166           
  Lines       52382    52382           
=======================================
  Hits        48395    48395           
  Misses       3987     3987
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <ø> (ø)` | :arrow_up: |
| #single | `42.92% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24810?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24810?src=pr&el=footer). Last update [17a6bc5...f8e5af0](https://codecov.io/gh/pandas-dev/pandas/pull/24810?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!