Build the test bootstrapper app as netcoreapp3.1 instead of netcoreapp2.0

[Numpsy]

I'm not sure what the intentions are for the bootstrapper app now that the github actions are running the tests directly, but:

I'm getting build warnings (visible in the appveyor build as well) about the bootstrapper app being built as .NET Core 2.0, when the test assembly that it depends on is now built as .NET Core 3.1 and .NET 4.6, so - try fixing that by retargetting it at 3.1 to match.

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[Numpsy]

Actually, looks like the appveyor CI is failing to run the tests altogether as is?

Invalid : C:/projects/sharpziplib/test/ICSharpCode.SharpZipLib.TestBootstrapper/bin/Debug/netcoreapp2/ICSharpCode.SharpZipLib.Tests.dll
Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information.
  ----> Could not load file or assembly 'System.ValueTuple, Version=4.0.3.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)