TST/DEPR: fix bunch of Panel deprecation warnings
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15965?src=pr&el=h1) Report
> Merging [#15965](https://codecov.io/gh/pandas-dev/pandas/pull/15965?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/838e09ccef896a1265b5447fb14f0969f7cd86d5?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15965/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/15965?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #15965   +/-   ##
=======================================
  Coverage      91%      91%           
=======================================
  Files         145      145           
  Lines       49581    49581           
=======================================
  Hits        45122    45122           
  Misses       4459     4459
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.77% <100%> (ø)` | :arrow_up: |
| #single | `40.57% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15965?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/types/missing.py](https://codecov.io/gh/pandas-dev/pandas/pull/15965?src=pr&el=tree#diff-cGFuZGFzL3R5cGVzL21pc3NpbmcucHk=) | `87.89% <100%> (ø)` | :arrow_up: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/15965?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `90.68% <0%> (-0.35%)` | :arrow_down: |
| [pandas/tseries/index.py](https://codecov.io/gh/pandas-dev/pandas/pull/15965?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvaW5kZXgucHk=) | `95.33% <0%> (-0.1%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/15965?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `81.13% <0%> (+0.18%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15965?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/15965?src=pr&el=footer). Last update [838e09c...bbe32e1](https://codecov.io/gh/pandas-dev/pandas/pull/15965?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15965?src=pr&el=h1) Report
> Merging [#15965](https://codecov.io/gh/pandas-dev/pandas/pull/15965?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/838e09ccef896a1265b5447fb14f0969f7cd86d5?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15965/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/15965?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15965      +/-   ##
==========================================
+ Coverage      91%   91.01%   +<.01%     
==========================================
  Files         145      145              
  Lines       49581    49581              
==========================================
+ Hits        45122    45124       +2     
+ Misses       4459     4457       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.77% <100%> (ø)` | :arrow_up: |
| #single | `40.57% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15965?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/types/missing.py](https://codecov.io/gh/pandas-dev/pandas/pull/15965?src=pr&el=tree#diff-cGFuZGFzL3R5cGVzL21pc3NpbmcucHk=) | `87.89% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/15965?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `81.13% <0%> (+0.18%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15965?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/15965?src=pr&el=footer). Last update [838e09c...71ffff5](https://codecov.io/gh/pandas-dev/pandas/pull/15965?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).