spec: let -tests subpackage own ostree-trivial-httpd
    
    We were missing another exclude for ostree-trivial-httpd. This was
    causing it to be shipped in the main ostree package instead, which in
    turned pulled libsoup back into AH.
    
    I kept the globby grab for everything else in
    `%{_libexecdir}/libostree}`, even though all the files we currently
    install in there are picked up by either the -grub2 or -tests
    subpackages.