wrap_test_suite: don't use --old-iso when testing branches based on Buster,...

wrap_test_suite: don't use --old-iso when testing branches based on Buster, until Tails 4.0 is released.

This can be reverted once 4.0 is released, but no big deal if we forget about it
for a while: the code will automatically revert to the "use --old-iso" behaviour
for Buster-based branches once 4.0 is out.