Edit docs to state that tests are ON by default

[anutosh491]

Found this while skimming through the docs

And we have it on by default (of for the emscripten case for now)

xeus-cpp/CMakeLists.txt

Line 62 in f054951

option(XEUS_CPP_BUILD_TESTS "xeus-cpp test suite" ON)

Can someone edit this saying On by default and currently switched off while building lite against emscripten.