Some tidying up

[hillalex]

• use testthat edition 3 everywhere
• use time_since_last_exp everywhere (instead of t_since_last_exp in places)
• make tests that run the model a bit faster by using fewer iterations
• replace RDS files of stan draws for tests with objects created using fit$save_object() rather than just saveRDS(fit) - the latter keeps references to temporary files that then get cleaned up and break the tests
• fix code in manual test files

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.48%. Comparing base (8155ae7) to head (09b42bb).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #24   +/-   ##
=======================================
  Coverage   92.48%   92.48%           
=======================================
  Files           8        8           
  Lines         426      426           
=======================================
  Hits          394      394           
  Misses         32       32           

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