TrustRegion: remove direct manipulation of sys.stdout / sys.__stdout__

[jsiirola]

Fixes # .

Summary/Motivation:

The trustregion tests directly manipulate sys.stdout -- and in particular -- restore it to the sys.__stdout__. This can foul up some testing / redirection logic, particularly if the test fails (raises an exception) before restoring the state. This PR updates the trustregion tests to use capture_output() instead of directly manipulating sys.stdout.

Changes proposed in this PR:

• Use the caputure_output() context manager instead of directly manipulating sys.stdout

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.93%. Comparing base (773f1d9) to head (fbb8413).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3902   +/-   ##
=======================================
  Coverage   89.92%   89.93%           
=======================================
  Files         902      902           
  Lines      106393   106393           
=======================================
+ Hits        95679    95681    +2     
+ Misses      10714    10712    -2     

Flag	Coverage Δ
builders	29.18% <ø> (+<0.01%)	⬆️
default	86.23% <ø> (?)
expensive	35.64% <ø> (?)
linux	87.38% <ø> (-2.04%)	⬇️
linux_other	87.38% <ø> (+<0.01%)	⬆️
oldsolvers	28.10% <ø> (+<0.01%)	⬆️
osx	82.71% <ø> (ø)
win	85.80% <ø> (ø)
win_other	85.80% <ø> (ø)

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