Use check-xeus-cpp target to run C++ tests for both native and Emscripten platform (+ make build instructions consistent across files)#386

mcbarton wants to merge 3 commits intocompiler-research:maincompiler-research/xeus-cpp:mainfrom

mcbarton:Make-running-C++-tests-the-check-xeus-cpp-targetmcbarton/xeus-cpp:Make-running-C++-tests-the-check-xeus-cpp-targetCopy head branch name to clipboard