Build: Fix tests w/ emulators that don't check CWD While QEMU will run executables from the current working directory, other emulators may not. It is more reliable to pass the full executable path to the emulator. The add_test(NAME ... COMMAND ...) syntax automatically invokes the emulator (e.g. the command specified in CMAKE_CROSSCOMPILING_EMULATOR) and passes the full executable path to it, as long as the first COMMAND argument is the name of a target. This cleans up the CMake code somewhat as well, since it is no longer necessary to manually invoke CMAKE_CROSSCOMPILING_EMULATOR. Closes #747