Add atomic slot migration test to validate DB and slot stats

[murphyjacob4]

This is just adds one more additional test, based on #2635 (will rebase onto unstable once that one is submitted)

[codecov]

Codecov Report

❌ Patch coverage is 89.25926% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.26%. Comparing base (8d562d2) to head (7adba33).
⚠️ Report is 1 commits behind head on unstable.

Files with missing lines	Patch %	Lines
src/cluster_migrateslots.c	88.93%	29 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #2638      +/-   ##
============================================
+ Coverage     72.22%   72.26%   +0.04%     
============================================
  Files           127      127              
  Lines         70936    71137     +201     
============================================
+ Hits          51232    51410     +178     
- Misses        19704    19727      +23     

Files with missing lines	Coverage Δ
src/cluster_legacy.c	87.28% <100.00%> (-0.03%)	⬇️
src/replication.c	86.02% <100.00%> (+0.04%)	⬆️
src/server.c	88.40% <100.00%> (+0.03%)	⬆️
src/cluster_migrateslots.c	91.78% <88.93%> (+0.08%)	⬆️

... and 11 files with indirect coverage changes

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