* Update monad-ext submodule with Xunit dlls
* Point Makefile xunit targets to monad-ext/xunit
* Add xunit.assert target to Makefile
* Add xunit.assert as dependency of ps_test.exe
* Add corerun as dependency of ps_test.exe
* Link ps_test.exe with xunit.assert
* Temporarily remove libps.so from ps_test.exe dependencies (WIP)
* Use Xunit in ps_test.cs
* Remove Assert mock
* Temporarily remove Platform.UserName (WIP)
With this patch, `make cleanall && make prepare && make run-test`
executes our (few) tests correctly using Xunit (without needing to `make
run` first).
