Test Formatting assemblies w/ `net6.0`

[dougbu]

• expand our text matrix to include a modern (and LTS) TFM
• move to VS2022 and the latest 6.0 .NET SDK for net6.0 support
• change how Formatting test assemblies are found
  • netcoreapp is no longer the only relevant folder prefix
• react to new Exception.Messages in netcoreapp3.1
  • handle different formatting of argument info in ArgumentException.Messages
  • handle slightly greater decimal precision in a JsonReaderException.Message
• react to new Exception.Messages and other changes in net6.0
  • handle different Message in InvalidOperationExceptions about invalid request URIs
  • handle HttpResponseMessage.Content != null after construction (a test-only change fortunately)
  • see dotnet/runtime@b48900f3b37e6d68b5d84a3e48f0c5beb602a128 which introduced this
  • handle inability to mock a Stream if a writer passes a ReadOnlySpan<byte> in net6.0 - see moq/moq4#829, moq/moq4#979, and dotnet/runtime#45152 about the issue
• nit: simplify define use now that NETCOREAPP3_1_OR_GREATER and so on are available

[dougbu]

/fyi successful runs on the CI will likely take a week because the v4.5 targeting pack is not available on VS2022 build agents right now

[dougbu]

Ran enough tests locally to confirm the last commit here does what it says on the tin. Will remove that commit and instead comment out broken tests and fix the infrastructure issue seen on the CI. Expect a new PR built on the "Add Content to all HttpResponseMessages" commit as well as a fix for ValidateStreamForReading(...)…

[dougbu]

/ping reviewers

I reset reviews because this works differently than the original PR. But it's working cleanly both locally and in the CI :grin:

Suggest reviewers pick and choose what they sign off on e.g.

• [ ] Updates to use binplaced msbuild from VS2022 on a VS2019 agent i.e. supporting net6.0 in msbuild while having old .NET Framework targeting packs available.
• [ ] Switch to net6.0 and related infrastructure changes
• [ ] Adjust tests to handle new netcoreapp3.1 and net6.0 behaviours
  • Note compiled-out tests will be restored (and fixed) in my next PR