refactor: read diff format configuration only once per test run

[haraldmaida]

Currently the diff format configuration for colored assertion failures is read from the environment variables once for each test case. This is not necessary. Reading the configuration once per test run is sufficient.

Refactored the assert_that function to read the configuration for the colored mode only once per test run.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.01%. Comparing base (e6b0182) to head (2a3ca88).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #27   +/-   ##
=======================================
  Coverage   94.01%   94.01%           
=======================================
  Files          24       24           
  Lines        2322     2323    +1     
  Branches     2322     2323    +1     
=======================================
+ Hits         2183     2184    +1     
  Misses        133      133           
  Partials        6        6           

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