Edited Task: MQ: Move xUnit build logic out into a NuGet package [1171]

Instead of having a custom, private copy of xUnit build system (test runner) logic (WebStack.xunit.targets), create a standard xUnit targets NuGet package (and share it with other projects such as EF). Also see if the xUnit owners would consider taking ownership of this package.

Note that the EF team is interested in this work as well and might be done in collaboration with them.