Add unit tests to cover primitive methods on QueryExpression types

[bkoelman]

Adds unit tests to improve coverage on types derived from QueryExpression, asserting on Equals, GetHashCode, ToString, etc.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.06%. Comparing base (cd8ae40) to head (a7e4225).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1751      +/-   ##
==========================================
+ Coverage   90.87%   92.06%   +1.19%     
==========================================
  Files         475      475              
  Lines       14703    14699       -4     
  Branches     2324     2322       -2     
==========================================
+ Hits        13361    13533     +172     
+ Misses        912      765     -147     
+ Partials      430      401      -29     

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