feat: implement soft assertions by providing new method Spec::soft_panic()

[haraldmaida]

Soft assertions are a useful feature of assertion libs. asserting should support soft assertions as well.

Implemented the new method Spec::soft_panic() to provide soft assertions. Soft assertions are started with either the macro verify_that! or the function verify_that(). This constructs a Spec with the failing strategy CollectFailures which collects the failures of multiple assertions instead of panicking immediately at the first failing assertion. To finally panic if at least one assertion failed, the method soft_panic() must be called after the last assertion.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.05%. Comparing base (67abdcc) to head (032cffd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #28      +/-   ##
==========================================
+ Coverage   94.01%   94.05%   +0.03%     
==========================================
  Files          24       24              
  Lines        2323     2338      +15     
  Branches     2323     2338      +15     
==========================================
+ Hits         2184     2199      +15     
  Misses        133      133              
  Partials        6        6              

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