Revert "CI: temp skip geopandas downstream tests (GH18456)"
Reverts pandas-dev/pandas#18457

Closes https://github.com/pandas-dev/pandas/issues/18456
Not sure why there is a commit in here trying to revert the move of the melt tests ... (I just used the github interface to revert PR #18457). But the diff looks ok # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18466?src=pr&el=h1) Report
> Merging [#18466](https://codecov.io/gh/pandas-dev/pandas/pull/18466?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e6eac0b308af9869ee123caa8c256bd8a7cc126b?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18466/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18466?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18466      +/-   ##
==========================================
- Coverage   91.35%   91.33%   -0.03%     
==========================================
  Files         163      163              
  Lines       49695    49695              
==========================================
- Hits        45401    45389      -12     
- Misses       4294     4306      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.11% <ø> (-0.03%)` | :arrow_down: |
| #single | `39.73% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18466?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18466?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.44% <0%> (-1.82%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18466?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/18466?src=pr&el=footer). Last update [e6eac0b...f5af11b](https://codecov.io/gh/pandas-dev/pandas/pull/18466?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
