Update Test Tooling to Officially Supported Framework#1

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Commits on Sep 9, 2017

Changed unit test tooling to xUnit - runners and test adapters are officially supported and maintained by the .Net Foundation for Visual Studio and the command line/CLI via dotnet test.
shealey
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Resolved missing xUnit dlls from publish output directory of FluentAssertions.AspNetCore.Mvc.Sample.Tests project
shealey
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