[Backend.Tests] Use Constraint Model in Helper, Models, and Services tests

[Apoktieno]

Closes #2444

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This change is 

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: +0.10% 🎉

Comparison is base (a7e7fbf) 67.91% compared to head (165203c) 68.02%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2466      +/-   ##
==========================================
+ Coverage   67.91%   68.02%   +0.10%     
==========================================
  Files         240      240              
  Lines        8273     8273              
  Branches      504      504              
==========================================
+ Hits         5619     5628       +9     
+ Misses       2298     2289       -9     
  Partials      356      356              

Flag	Coverage Δ
backend	79.76% <ø> (+0.20%)	⬆️
frontend	54.31% <ø> (ø)

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see 3 files with indirect coverage changes

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[imnasnainaec]

Reviewed 14 of 17 files at r1, 2 of 4 files at r2, all commit messages.
Reviewable status: 16 of 18 files reviewed, 2 unresolved discussions (waiting on @Apoktieno)

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Backend.Tests/Services/MergeServiceTests.cs line 196 at r2 (raw file):

            Assert.That(frontierWordIds, Does.Contain(childIds[0]));
            Assert.That(frontierWordIds, Does.Contain(childIds[1]));
            Assert.That(frontierWordIds, Does.Contain(childIds[2]));

Let's use a childIds.ForEach( loop here, so we don't have to explicitly [0], [1], and [2].

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Backend.Tests/Services/StatisticsServiceTests.cs line 98 at r2 (raw file):

            var result = _statsService.GetSemanticDomainCounts(ProjId, "").Result;
            Assert.That(result, Has.Count.EqualTo(1));
            Assert.That(result.First(), Has.Count.EqualTo(0));

Could this one be Is.Empty?

[imnasnainaec]

Reviewed 2 of 2 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @Apoktieno)