Test using the .NET 6 SDK and stop adding a duplicate compile item for WPF

[jnm2]

Fixes #89 (CI doesn't show it till we fix some unrelated things, but running the test locally does)

The [research needed] commit is unrelated to #89 and looks like it will be annoying to figure out.

[jnm2]

@sharwell ArrayDimension is a struct coming from the Mono.Cecil package. This issue doesn't repro unless RAA is rewriting reference assemblies.

Not sure if this is an existing RAA problem when used normally or whether it's a regression introduced in this PR.

[jnm2]

This build failure is surprising, will have to come back and look closer when I have time to guess at what caused it.

dotnet msbuild -restore tests/NoDisabledWarnings -warnaserror -nr:false -v:m
Microsoft (R) Build Engine version 17.0.0+c9eb9dd64 for .NET
Copyright (C) Microsoft Corporation. All rights reserved.
  Determining projects to restore...
  Restored C:\projects\referenceassemblyannotator\tests\NoDisabledWarnings\NoDisabledWarnings.csproj (in 6.57 sec).
C:\projects\referenceassemblyannotator\tests\.isolatednugetcache\tunnelvisionlabs.referenceassemblyannotator\1.0.0-alpha.172.ga01117b533\build\TunnelVisionLabs.ReferenceAssemblyAnnotator.targets(123,5): error RA1000: Skipping mixed-mode implementation assembly 'System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' [C:\projects\referenceassemblyannotator\tests\NoDisabledWarnings\NoDisabledWarnings.csproj]