Resolve capture_output deadlock in multiprocessing environments

[jsiirola]

Fixes # .

Summary/Motivation:

#3908 pointed out that we could inadvertently trigger a deadlock in multiprocessing environments. This PR attempts to resolve that by switching from a threading.Lock to a multiprocessing.Lock if multiprocessing is being used in the python process.

I am not closing the issue as the original reporters have not confirmed if it actually resolves their use case.

As part of this, I needed to overhaul the import time test / reporting logic so that we can more accurately test import timing for multiprocessing environments.

Changes proposed in this PR:

• Switch to a multiprocessing.Lock when multiprocessing is imported
• Make sure that multiprocessing is not imported by pyomo.environ
• Overhaul (and generalize) the import time reporting logic

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

❌ Patch coverage is 80.64516% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.28%. Comparing base (6ffd3cb) to head (5e7ad72).

Files with missing lines	Patch %	Lines
pyomo/common/unittest.py	25.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3957      +/-   ##
==========================================
- Coverage   90.10%   88.28%   -1.82%     
==========================================
  Files         908      908              
  Lines      108344   108360      +16     
==========================================
- Hits        97620    95668    -1952     
- Misses      10724    12692    +1968     

Flag	Coverage Δ
builders	29.12% <58.06%> (+<0.01%)	⬆️
default	86.08% <80.64%> (?)
expensive	35.14% <58.06%> (?)
linux	34.55% <58.06%> (-55.06%)	⬇️
linux_other	34.55% <58.06%> (-53.07%)	⬇️
oldsolvers	28.07% <58.06%> (+<0.01%)	⬆️
osx	?
win	?

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