Removing dependency on JsonApiDotNetCoreExample project in test projects

[maurei]

Closes #1031

Also required for introducing a OpenAPI testing environment in #1047.

[codecov]

Codecov Report

Merging #1068 (4a38b27) into master (a8a0bf0) will decrease coverage by 1.46%.
The diff coverage is n/a.

❗ Current head 4a38b27 differs from pull request most recent head 19a3c51. Consider uploading reports for the commit 19a3c51 to get more accurate results

@@            Coverage Diff             @@
##           master    #1068      +/-   ##
==========================================
- Coverage   89.95%   88.48%   -1.47%     
==========================================
  Files         257      256       -1     
  Lines        7026     7024       -2     
==========================================
- Hits         6320     6215     -105     
- Misses        706      809     +103     

Impacted Files	Coverage Δ
...mples/JsonApiDotNetCoreExample/Startups/Startup.cs	0.00% <ø> (-95.84%)	⬇️
...ples/JsonApiDotNetCoreExample/Data/AppDbContext.cs	0.00% <0.00%> (-100.00%)	⬇️
...NetCoreExample/Controllers/NonJsonApiController.cs	0.00% <0.00%> (-100.00%)	⬇️
...NetCoreExample/Controllers/OperationsController.cs	0.00% <0.00%> (-100.00%)	⬇️
src/Examples/JsonApiDotNetCoreExample/Program.cs	0.00% <0.00%> (-66.67%)	⬇️
...iDotNetCore/Diagnostics/DefaultCodeTimerSession.cs	0.00% <0.00%> (-60.00%)	⬇️
...sonApiDotNetCore/Diagnostics/CascadingCodeTimer.cs	0.00% <0.00%> (-10.19%)	⬇️
...DotNetCore/Configuration/ServiceDiscoveryFacade.cs	97.93% <0.00%> (-2.07%)	⬇️
...piDotNetCore/Configuration/ResourceGraphBuilder.cs	97.56% <0.00%> (-1.22%)	⬇️

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