Adapt STI tests to current meson
    
    Meson now requires a build directory out of source tree.
    
    Setting STI dir to "{{ tenv_workdir }}source" does not work. It
    expands to "/var/str/var/str/source" instead of "var/str/source".
    Hence this patch passes to meson an explicit path to the directory
    with libmodulemd sources.