Unit test platform updates

[waf]

A bit of modernization in the tests:

• xunit 2 to xunit.v3.mtp-v2 + Microsoft Testing Platform changes
  • There is a new warning to solve, xUnit1051. I put that in a separate commit.
• coverlet -> Microsoft.Testing.Extensions.CodeCoverage
• remove need to set DOTNET_CLI_USE_MSBUILD_SERVER / MSBUILDDISABLENODEREUSE
  • I think this is no longer required given the move the MTPv2 + I cleaned up some console input redirection issues in the unit tests. These redirection issues might have been the real root cause for hanging tests.
• Migrate to slnx

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 71.2%. Comparing base (1a8f551) to head (dd9308c).

Files with missing lines	Patch %	Lines
CSharpRepl/Program.cs	87.5%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #456     +/-   ##
=======================================
+ Coverage   70.8%   71.2%   +0.3%     
=======================================
  Files         90      94      +4     
  Lines       6834    6793     -41     
  Branches     937    1003     +66     
=======================================
- Hits        4842    4837      -5     
+ Misses      1640    1626     -14     
+ Partials     352     330     -22     

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