Fix flaky presigned upload tests

[MrCreosote]

minio.get_object() initiates a client close before returning the object data, which means obj["Body"].read() may or may not fail if the read occurs after the client is closed.

Get rid of get_object() and use the client directly in the tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.33%. Comparing base (b8dff75) to head (861725f).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #579   +/-   ##
=======================================
  Coverage   53.33%   53.33%           
=======================================
  Files          69       69           
  Lines        5975     5975           
=======================================
  Hits         3187     3187           
  Misses       2788     2788           

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