Add more tests for MimePartMatcher

[StefH]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 78.78788% with 42 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.29%. Comparing base (f5d5345) to head (041d16e).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/WireMock.Net.Shared/Matchers/MatchResult.cs	60.60%	13 Missing ⚠️
.../WireMock.Net.Minimal/Matchers/CompositeMatcher.cs	75.60%	10 Missing ⚠️
...tchers/Request/RequestMessageHttpVersionMatcher.cs	14.28%	6 Missing ⚠️
...Matchers/Request/RequestMessageMultiPartMatcher.cs	40.00%	3 Missing ⚠️
...d/Matchers/Helpers/BodyDataMatchScoreCalculator.cs	85.00%	3 Missing ⚠️
...al/Matchers/Request/RequestMessageBodyMatcher`1.cs	60.00%	2 Missing ⚠️
.../Matchers/Request/RequestMessageClientIPMatcher.cs	60.00%	2 Missing ⚠️
...nimal/Matchers/Request/RequestMessageUrlMatcher.cs	50.00%	2 Missing ⚠️
...al/Matchers/Request/RequestMessageCookieMatcher.cs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1389      +/-   ##
==========================================
+ Coverage   34.84%   35.29%   +0.45%     
==========================================
  Files         153      154       +1     
  Lines       36137    40800    +4663     
==========================================
+ Hits        12593    14402    +1809     
- Misses      23106    25861    +2755     
- Partials      438      537      +99     

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