Bluetooth: host: add `prj.conf` for ATT illegal pipeline test with rx/tx priorities inverted

[alwa-nordic]

@alwa-nordic can you add a prj.conf for the pipeline test with the priorities inverted? can be in a future PR

Originally posted by @jori-nordic in #71391 (comment)

tests/bsim/bluetooth/host/att/pipeline

[jori-nordic]

we need a bot for this kinda stuff lol