Use tmpdir in callback and auxiliary tests

[ChrisspyB]

Fixes potential race condition when these tests are run in parallel (e.g. fdb.wipe deleting data that is being used by another test)

[codecov-commenter]

Codecov Report

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

Project coverage is 64.15%. Comparing base (1001d61) to head (b0ef4d3).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #43      +/-   ##
===========================================
+ Coverage    64.07%   64.15%   +0.08%     
===========================================
  Files          238      238              
  Lines        13623    13629       +6     
  Branches      1312     1312              
===========================================
+ Hits          8729     8744      +15     
+ Misses        4894     4885       -9     

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