test: Use XCTestExpectation for FlushTests

[philipphofmann]

Replace DispatchGroup with XCTextExpectation in
SentryHttpTransportFlushIntegrationTests.

Contributes to #5789

#skip-changelog

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.672%. Comparing base (8d944ac) to head (52aa1f7).
⚠️ Report is 4 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #5903       +/-   ##
=============================================
- Coverage   86.686%   86.672%   -0.014%     
=============================================
  Files          423       423               
  Lines        36527     36527               
  Branches     17281     17278        -3     
=============================================
- Hits         31664     31659        -5     
- Misses        4815      4821        +6     
+ Partials        48        47        -1     

see 6 files with indirect coverage changes

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[philprime]

LGTM