TST: move melt tests to separate file
- [x] closes #18367

This breaks out melt, lreshape, and wide_to_long tests into their own module.
can you rebase this. ping as soon as green to avoid future conflict. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18428?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@b45325e`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18428/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18428?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #18428   +/-   ##
=========================================
  Coverage          ?   91.31%           
=========================================
  Files             ?      163           
  Lines             ?    49714           
  Branches          ?        0           
=========================================
  Hits              ?    45394           
  Misses            ?     4320           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.1% <ø> (?)` | |
| #single | `39.63% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18428?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/18428?src=pr&el=footer). Last update [b45325e...54226dd](https://codecov.io/gh/pandas-dev/pandas/pull/18428?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/18428?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@b45325e`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18428/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18428?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #18428   +/-   ##
=========================================
  Coverage          ?   91.31%           
=========================================
  Files             ?      163           
  Lines             ?    49714           
  Branches          ?        0           
=========================================
  Hits              ?    45394           
  Misses            ?     4320           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.1% <ø> (?)` | |
| #single | `39.63% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18428?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/18428?src=pr&el=footer). Last update [b45325e...54226dd](https://codecov.io/gh/pandas-dev/pandas/pull/18428?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback when you say 'ping on green' do you mean wait for the three continuous integration services to finish or that big green circle with a checkmark that says 'this branch has no conflicts with the bae branch'?  the ci to be all green (may take a while) @jreback its green now (that took 4 hours?). Looks like there has been another commit to master. Should i rebase again? yeah that’s fine- the commit to master fixes the ci issue
no need to rebase 

ci runs for every commit for all prs so quite a queue thanks 