[SCHEMA] Add internal validation test for schema

[tsalo]

References #1037.

Changes proposed:

• Add a test that checks that all objects referenced in the schema rules are also defined as schema objects.

To do:

• [ ] Add test for order of entity definitions in objects/entities.yaml (#1071).

[codecov[bot]]

Codecov Report

Merging #1058 (d86f779) into master (e140f35) will increase coverage by 1.65%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1058      +/-   ##
==========================================
+ Coverage   70.53%   72.19%   +1.65%     
==========================================
  Files           9       10       +1     
  Lines         930      953      +23     
==========================================
+ Hits          656      688      +32     
+ Misses        274      265       -9     

Impacted Files	Coverage Δ
tools/schemacode/schemacode/tests/test_rules.py	100.00% <100.00%> (ø)
tools/schemacode/schemacode/_version.py	38.90% <0.00%> (+2.59%)	:arrow_up:

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[Remi-Gau]

Related to #1193