The documentation states that users can override where to take a
dependency from via variables like VULKAN_HEADERS_INSTALL_DIR, however
the helper.cmake written by the scripts/CMakeLists.txt FORCE updates
the cache variable to the location inside known_good.json, practically
overwriting the user-provided path given on the CLI using -D, also
going into the cache.
Without going through an intricate dance with git to change the remote
URL of a repository, it's simplest to just nuke the cloned folder, as
we have to do a clean clone anyway.
Pulling in latest changes from VVL
Use BUILD_TESTS instead of VUL_TESTS. While it's good practice to
prefix variable names it makes updating this code more difficult.
Also since tests can only ever be enabled if the project is top
level, it doesn't affect add_subdirectory users.
Other misc. CMake cleanup
