TST: Move merge/join tests into sub-dir

[reidy-p]

• closes TST: split out merge/join tests to a sub-dir #18615

[codecov]

Codecov Report

Merging #18643 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #18643      +/-   ##
==========================================
- Coverage    91.6%   91.58%   -0.02%     
==========================================
  Files         153      153              
  Lines       51253    51253              
==========================================
- Hits        46950    46941       -9     
- Misses       4303     4312       +9

Flag	Coverage Δ
#multiple	89.45% <ø> (ø)	⬆️
#single	40.67% <ø> (-0.11%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.81% <0%> (-0.1%)	⬇️

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[jreback]

thanks @reidy-p