tests: Bluetooth: tester: Workaround Bus Fault in nRF53x using full RAM

Workaround failing autopts weekly run due to tester
application having Bus Fault in the nRF53 NET core hci_ipc
firmware at startup.

Having atleast 32 bytes free in nRF53x hci_ipc sample to
avoid Bus Fault when Zephyr Kernel does SYSINIT calls.

Reduce supported ISO Broadcaster instance and ISO TX buffer
counts in the Controller hci_ipc firmware.

Add sysbuild support for tester application so that the
build is CI verified.

sysbuild commandline for LE audio PTS testing:

  # west build --sysbuild -b nrf5340_audio_dk/nrf5340/cpuapp
    -d build/tester tests/bluetooth/tester
    -DEXTRA_CONF_FILE="overlay-le-audio.conf"

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>