Add Xunit.Runner.VisualStudio to test projects so the VS Addin is not needed anymore #210
Description
Right now the test projects contain only the Xunit package/reference. So we need the Xunit runner vsix to detect the tests.
Right now the recommended way is to use Xunit.Runner.VisualStudio, so that package/reference should be added to all test projects on the Roslyn solution.