test(expect): improve expect test cases

[kt3k]

This PR adds test cases of:

• expect().resolves
• expect().rejects
• Throwing cases of custom matchers.

This improves the code coverage of expect about 2%.

part of #3713

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.59%. Comparing base (d6c3170) to head (ecb79b7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5221      +/-   ##
==========================================
+ Coverage   94.49%   94.59%   +0.10%     
==========================================
  Files         467      467              
  Lines       38143    38143              
  Branches     5494     5502       +8     
==========================================
+ Hits        36042    36082      +40     
+ Misses       2062     2023      -39     
+ Partials       39       38       -1     

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