Defer pathlib import, remove test output file

[jsiirola]

Fixes # .

Summary/Motivation:

A recent PR (#3478) accidentally added a test output file to the repository and made it so that pathlib was now being importe through pyomo.environ. The latter as caused some Jenkins tests that monitor import time to fail. This PR removed the test output file and adds pathlib to the set of deferred import modules in pyomo.common.dependencies

Changes proposed in this PR:

• Add pathlib to pyomo.common.dependencies
• Remove a test output file (test4.soln)

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

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

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.73%. Comparing base (0b58294) to head (aca625e).
Report is 37 commits behind head on main.

Files with missing lines	Patch %	Lines
...trib/doe/examples/reactor_compute_factorial_FIM.py	50.00%	1 Missing ⚠️
pyomo/contrib/doe/examples/reactor_example.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3563      +/-   ##
==========================================
- Coverage   88.73%   88.73%   -0.01%     
==========================================
  Files         890      890              
  Lines      102194   102192       -2     
==========================================
- Hits        90682    90677       -5     
- Misses      11512    11515       +3     

Flag	Coverage Δ
builders	26.62% <42.85%> (+<0.01%)	⬆️
default	84.84% <57.14%> (?)
expensive	34.01% <42.85%> (?)
linux	86.24% <71.42%> (-2.24%)	⬇️
linux_other	86.24% <71.42%> (+<0.01%)	⬆️
osx	76.19% <57.14%> (+<0.01%)	⬆️
win	84.74% <57.14%> (-0.01%)	⬇️
win_other	84.74% <57.14%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.