Update aspnetcore50 dependencies
Add aspnetcore50 to the abstractions and core NUSPEC files
Introduce ITest to wrap test case + test-specific display name.
Add ITestOutput (and ITestOutputHelper to generate test output)
Add IDiagnosticMessage for post beta-5 configuration support
Handle case when stack trace is not in the v1 XML
Missed adding aspnetcore50 to xunit.assert.nuspec
Update TestMessageVisitor for IDiagnosticMessage