[TST] make xfails strict
This leaves untouched xfails with reasons like "flaky on 3.7" and a few where the test itself is just a `pass`.

In the process a handful of previously xfailed cases are no longer xfailed.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22139?src=pr&el=h1) Report
> Merging [#22139](https://codecov.io/gh/pandas-dev/pandas/pull/22139?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c272c52a548d4b75d32857d8b33821a3b0b41187?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/22139/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/22139?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22139      +/-   ##
==========================================
- Coverage   92.07%   92.06%   -0.01%     
==========================================
  Files         170      170              
  Lines       50688    50689       +1     
==========================================
- Hits        46671    46669       -2     
- Misses       4017     4020       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.47% <ø> (-0.01%)` | :arrow_down: |
| #single | `42.29% <ø> (-0.03%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22139?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/22139/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `94.87% <0%> (-0.34%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/22139/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.69% <0%> (-0.21%)` | :arrow_down: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/22139/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `95.44% <0%> (-0.03%)` | :arrow_down: |
| [pandas/core/dtypes/dtypes.py](https://codecov.io/gh/pandas-dev/pandas/pull/22139/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2R0eXBlcy5weQ==) | `96.05% <0%> (+0.02%)` | :arrow_up: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/22139/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `93.5% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22139?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/22139?src=pr&el=footer). Last update [c272c52...67ab350](https://codecov.io/gh/pandas-dev/pandas/pull/22139?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks, ideally also update the  contributing.rst docs where we use xfail as an example to include the strict param.