fix: cache df in synthetic diff in differences nb test

[BrendanWalsh]

cache the df in synthdiff to prevent flaky test failures

[BrendanWalsh]

/azp run

[azure-pipelines[bot]]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.61%. Comparing base (6c95bf0) to head (1825862).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2369      +/-   ##
==========================================
- Coverage   84.64%   84.61%   -0.04%     
==========================================
  Files         331      331              
  Lines       17180    17180              
  Branches     1544     1544              
==========================================
- Hits        14542    14536       -6     
- Misses       2638     2644       +6     

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[mhamilton723]

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).