Avoid reuse of a global buffer between test cases

[guhetier]

Description

QuicTestStreamMultiReceive was using a global buffer as a send buffer, shared between test cases.
Test cases would modify the buffer, but also assume it was zeroed.

Testing

CI

Documentation

N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.24%. Comparing base (9be1d72) to head (2727867).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4937      +/-   ##
==========================================
- Coverage   87.45%   86.24%   -1.21%     
==========================================
  Files          56       56              
  Lines       17723    17723              
==========================================
- Hits        15499    15286     -213     
- Misses       2224     2437     +213     

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