Separate UtilTests.py into individual files

[Repiteo]

• Split from Split SCons.Util even more, discourage implicit imports #4830

This consolidates the Util test file separation into its own PR, as I'm no longer wholly convinced that the original PR is the right approach for "deprecating" imports.

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt and RELEASE.txt (and read the README.rst).
• I have updated the appropriate documentation

[mwichmann]

Okay, that was weird stuff from one of the appveyor instances:

.\.appveyor\install.ps1
Python version check failed or Python not found at C:\Python312\python.exe. Installing Python312 via Chocolatey...
Chocolatey v0.10.15
Installing the following packages:
Python312
By installing you accept licenses for the packages.
python312 v3.12.10 already installed.
 Use --force to reinstall, specify a version to install, or try upgrade.
Chocolatey installed 0/1 packages. 
 See the log for details (C:\ProgramData\chocolatey\logs\chocolatey.log).
Warnings:
 - python312 - python312 v3.12.10 already installed.
 Use --force to reinstall, specify a version to install, or try upgrade.
& : The term 'C:\Python312\python.exe' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included, verify that the path is correct and try 
again.
At C:\projects\scons\.appveyor\install.ps1:22 char:20
+ $env:PYSITEDIR = & $pythonExe -c "import sys; print(sys.path[-1])"
+                    ~~~~~~~~~~
    + CategoryInfo          : ObjectNotFound: (C:\Python312\python.exe:String) [], CommandNotFoundException
    + FullyQualifiedErrorId : CommandNotFoundException

Don't find 3.12, install 3.12 - but it's already installed, don't find 3.12?