TST: move melt tests to separate file

[tdpetrou]

• closes TST: split out melt-tests in pandas/tests/reshape/test_reshape.py #18367

This breaks out melt, lreshape, and wide_to_long tests into their own module.

[jreback]

can you rebase this. ping as soon as green to avoid future conflict.

[codecov]

Codecov Report

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@@            Coverage Diff            @@
##             master   #18428   +/-   ##
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  Coverage          ?   91.31%           
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  Branches          ?        0           
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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@b45325e). Click here to learn what that means.
The diff coverage is n/a.

@@            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.

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[tdpetrou]

@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'?

[jreback]

the ci to be all green (may take a while)

[tdpetrou]

@jreback its green now (that took 4 hours?). Looks like there has been another commit to master. Should i rebase again?

[jreback]

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

[jreback]

thanks