dotnet test help message and docs need to be updated to reflect test and vstest verb convergence

[ViktorHofer]

With https://github.com/dotnet/toolset/commit/1214ec9da622b4a751230a6c2240b1c103791728#diff-3e71c85821fdc972adec6f9e60e2541e, dotnet test now also supports invoking an assembly which makes dotnet vstest de facto obsolete. The docs and dotnet test help message needs to be updated to reflected the added feature.