test: Use XCTUnwrap in SerializationTests

[philipphofmann]

Replace if conditions or guard statements with XCTUnwrap.

#skip-changelog

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.683%. Comparing base (888a145) to head (fc00e73).
Report is 1 commits behind head on main.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #4341       +/-   ##
=============================================
+ Coverage   91.641%   91.683%   +0.041%     
=============================================
  Files          619       620        +1     
  Lines        50298     50382       +84     
  Branches     18092     18169       +77     
=============================================
+ Hits         46094     46192       +98     
+ Misses        4112      4098       -14     
  Partials        92        92               

Files with missing lines	Coverage Δ
.../SentryTests/Helper/SentrySerializationTests.swift	99.528% <100.000%> (+0.923%)	⬆️

... and 17 files with indirect coverage changes

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