add wrapping script for testing with frame/tile-threads
Use the wrapping capabilities of meson test.
meson test --wrap ../tests/dav1d-test-data/wrap_mt.sh
will append
the values of the environment variables FRAMETHREADS and TILETHREADS
as '--framethreads' and '--tilethreads' command line parameters.
If the varables are not set it will default to 2 frame and tile threads.