feat: do not print "assertion failed" with each failing assertion

[haraldmaida]

Removes the phase "assertion failed: " from the message of failing assertions.

This phrase does not add much the clarity of a failing test. If one is running tests, it is obvious that panic messages most likely are about failing assertions.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.91%. Comparing base (3198000) to head (ebec143).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spec/mod.rs	0.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #65   +/-   ##
=======================================
  Coverage   93.91%   93.91%           
=======================================
  Files          32       32           
  Lines        4438     4438           
  Branches     4438     4438           
=======================================
  Hits         4168     4168           
  Misses        233      233           
  Partials       37       37           

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