Add minimal pytest plugin to verify that generated files are cleaned up by tests

[lbianchi-lbl]

Summary/Motivation:

• @andrewlee94 discovered that a conspicuous number of untracked files are generated when running the test suite
• I was able to confirm his intuition that this was most likely a regression introduced by Remove idaes.core.dmf and related code #1484
• It's not entirely clear how exactly the presence of the DMF code was causing generated files not to be saved when running test modules in different parts of the codebase
• My best guess (mostly based on gut feeling, i.e. without any rigorous investigation) is that it might have something to do with the code formerly in idaes.core.dmf.util through manipulation of temporary directories and/or the working directory

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 74.11765% with 22 lines in your changes missing coverage. Please review.

Project coverage is 76.83%. Comparing base (b6be02e) to head (33edfd6).
Report is 23 commits behind head on main.

Files with missing lines	Patch %	Lines
idaes/conftest.py	73.80%	20 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1488      +/-   ##
==========================================
- Coverage   76.84%   76.83%   -0.02%     
==========================================
  Files         376      376              
  Lines       61091    61170      +79     
  Branches    13505    13518      +13     
==========================================
+ Hits        46948    46999      +51     
- Misses      11762    11788      +26     
- Partials     2381     2383       +2     

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[ksbeattie]

LGTM