This will make test/ directory structure consistent with new example/ directory. I want the example structure to be as simple as possible, and there's no need for tests or examples to have separate include/ and src/ subdirectories when they aren't installing public headers to /usr/include.
Also get rid of BUILD_INTERFACE cmake expressions in mptest target include directory list for the same reason. mptest isn't an installed library so it has no need for differentiated BUILD_INTERFACE and INSTALL_INTERFACE include lists (https://cmake.org/cmake/help/latest/command/target_include_directories.html).