BT_AUDIO_UNICAST selection rejected in nightly tests

[pabigot]

I don't know if this is going to cause a nightly failure, but I suspect it will/should:

These tests:

• tests/bluetooth/init/bluetooth.init.test_ctlr_peripheral_iso
• tests/bluetooth/init/bluetooth.init.test_ctlr_central_iso

on these platforms:

• nrf51dk_nrf51422
• nrf52840dk_nrf52840

all appear to produce this diagnostic:

warning: BT_AUDIO_UNICAST (defined at subsys/bluetooth/host/audio/Kconfig:25) was assigned the value
'y' but got the value 'n'. Check these unsatisfied dependencies: BT_AUDIO (=n). See
http://docs.zephyrproject.org/latest/reference/kconfig/CONFIG_BT_AUDIO_UNICAST.html and/or look up
BT_AUDIO_UNICAST in the menuconfig/guiconfig interface. The Application Development Primer, Setting
Configuration Values, and Kconfig - Tips and Best Practices sections of the manual might be helpful
too.

The latest nightly still fails because it ran before #32013 got merged, but although the build completes on current mainline the Kconfig diagnostic is still present.

[pabigot]

@nashif @galak @carlescufi fyi for triage purposes.

[pabigot]

FWIW: https://buildkite.com/zephyr/zephyr-daily/builds/232 shows nightly passes now, however, the warning above is still observed when building the sample manually with the corresponding prj_ctlr_peripheral_iso.conf. This seems like a flaw in CI, especially in cases like this where it seems the test passes because it quietly excludes the feature it's supposed to test. @nashif