Start using %meson_test again instead of %__meson
    
    Since meson 0.51.0 %meson_test has been changed so that it invokes tests
    through meson instead of ninja, which allows us to pass
    --timeout-multiplier directly to %meson_test. Since meson 0.51.0 has
    made it into Fedora 29 and 30 finally, stop using %__meson and bump our
    meson requirement to 0.51.0.