TST: Use fixtures in dtypes/test_cast.py
Title is self-explanatory.

Partially addresses #21500.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21661?src=pr&el=h1) Report
> Merging [#21661](https://codecov.io/gh/pandas-dev/pandas/pull/21661?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a620e7255ce896c9185275d82172513268b1a719?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/21661/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21661?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21661   +/-   ##
=======================================
  Coverage    91.9%    91.9%           
=======================================
  Files         154      154           
  Lines       49555    49555           
=======================================
  Hits        45542    45542           
  Misses       4013     4013
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.27% <ø> (ø)` | :arrow_up: |
| #single | `42.03% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21661?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/21661?src=pr&el=footer). Last update [a620e72...b2ade3c](https://codecov.io/gh/pandas-dev/pandas/pull/21661?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @gfyoung 