Consolidate testing guidelines

There are multiple documents under https://github.com/dotnet/coreclr/tree/master/Documentation/building that describe how testing can be accomplished. These docs include building and running tests. However since we are using platform agnostic tools, the instructions should also be largely agnostic of platform. Minor command line differences, but that are basically limited to use the .sh vs .cmd file. There is a Workflow section under Documentation that has a placeholder file for this type of information - RunningTests.md.