[core] Correctly set _sourceCodeFolder in BaseNode's constructor

[cbentejac]

Description

This PR fixes a regression introduced by the refactoring of the command line variables in #2888, where the static nodeSourceCodeFolder is initialized but never set with the correct value once it becomes available.

This is fixed by initializing _sourceCodeFolder directly in the BaseNode's constructor if the description is available (if it isn't, the node will be in Unknown Node compatibility error anyway). This both simplifies what was done prior to #2888 (where _sourceCodeFolder was correctly set later than it should have been) and ensures that _staticExpVars["nodeSourceCodeFolder"] is initialized with the correct value as well.

Additionally, simple unit tests are added to ensure that static variables are always correctly set. A test InputNode is also added to one of the test plugins to cover all cases.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.86%. Comparing base (e88f34a) to head (e8ae796).
⚠️ Report is 36 commits behind head on develop.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2910      +/-   ##
===========================================
+ Coverage    80.61%   80.86%   +0.24%     
===========================================
  Files           55       57       +2     
  Lines         7692     7792     +100     
===========================================
+ Hits          6201     6301     +100     
  Misses        1491     1491              

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