TST: Parameterize more tests
Add parameterization to the following tests:

* `frame/test_dtypes.py`
* `series/indexing/test_boolean.py`
* `reshape/merge/test_merge_asof.py`
lgtm. merge on green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21873?src=pr&el=h1) Report
> Merging [#21873](https://codecov.io/gh/pandas-dev/pandas/pull/21873?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/bdb61688aa80a6ec9bdb6832946dd83ab4b4c9c8?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21873/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21873?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21873      +/-   ##
==========================================
- Coverage   91.91%   91.91%   -0.01%     
==========================================
  Files         164      164              
  Lines       49992    49992              
==========================================
- Hits        45952    45951       -1     
- Misses       4040     4041       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (-0.01%)` | :arrow_down: |
| #single | `42.16% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21873?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/21873/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.1% <0%> (-0.16%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21873?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/21873?src=pr&el=footer). Last update [bdb6168...d9a3315](https://codecov.io/gh/pandas-dev/pandas/pull/21873?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Finally green!  Merging.