Move some tests from core to expr

[andygrove]

Which issue does this PR close?

Part of #2683

Rationale for this change

Some tests got left behind during the recent refactoring

What changes are included in this PR?

Move some tests to live with the code they are testing

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #2700 (0d748b6) into master (9306534) will increase coverage by 0.00%.
The diff coverage is 94.82%.

@@           Coverage Diff           @@
##           master    #2700   +/-   ##
=======================================
  Coverage   84.66%   84.66%           
=======================================
  Files         270      270           
  Lines       46919    46919           
=======================================
+ Hits        39724    39726    +2     
+ Misses       7195     7193    -2     

Impacted Files	Coverage Δ
datafusion/core/src/logical_plan/plan.rs	84.61% <ø> (-9.19%)	⬇️
datafusion/expr/src/logical_plan/plan.rs	74.75% <94.82%> (+6.79%)	⬆️
datafusion/optimizer/src/utils.rs	32.25% <0.00%> (-0.54%)	⬇️
datafusion/core/src/physical_plan/metrics/value.rs	86.93% <0.00%> (-0.51%)	⬇️
datafusion/expr/src/window_frame.rs	93.27% <0.00%> (+0.84%)	⬆️

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