[rel/3.8] Fix serializing special characters in Jsonite

[youssef-backport-bot]

Backport of #5124 to rel/3.8

/cc @Evangelink @nohwnd

[Youssef1313]

@nohwnd Build is failing here. Want to take a look?

[nohwnd]

I've simplified the test, in 3.9 we get System.Text.Json available in .NET Framework thanks to updated dependency on Code Coverage, which updaged from version that used Newtonsoft.Json.

S:\p\testfx [backport/5124/to/rel/3.8 ≡]> dotnet nuget why S:\p\testfx\test\UnitTests\Microsoft.Testing.Platform.UnitTests\Microsoft.Testing.Platform.UnitTests.csproj system.text.json
Project 'Microsoft.Testing.Platform.UnitTests' has the following dependency graph(s) for 'system.text.json':

  [net6.0/win-x86]
  [net6.0]
   │
   └─ Microsoft.Testing.Extensions.CodeCoverage (v17.13.1)
      └─ Microsoft.Extensions.DependencyModel (v6.0.1)
         └─ System.Text.Json (v6.0.10)

  [net462/win-x86]
  [net462]
  [net7.0/win-x86]
  [net7.0]
  [net8.0/win-x86]
  [net8.0]
  [net9.0/win-x86]
  [net9.0]
   │
   └─ No dependency graph(s) found for this target framework.

[nohwnd]

/azp run