test(attachments): cleanup scope flush assertions

[JoshuaMoelans]

follow-up to #1934 , more robustly fetching the keys using get() and folded the assertion into the function that we wait for. No longer should fail on unflushed content_type as in https://github.com/getsentry/sentry-native/actions/runs/30884798825/job/91913533861#step:31:63880

#skip-changelog

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.87%. Comparing base (f537fa2) to head (98e61c0).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1947      +/-   ##
==========================================
- Coverage   75.90%   75.87%   -0.04%     
==========================================
  Files          93       93              
  Lines       22183    22183              
  Branches     3951     3951              
==========================================
- Hits        16838    16831       -7     
- Misses       4459     4467       +8     
+ Partials      886      885       -1     

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[jpnurmi]

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