Bluetooth: tests: Include building mesh ext. in CI

Include the HCI Mesh Extension implementation to be built in
CI. Note, mesh extension implementation is work in progress,
and will be continues in new LL split architecture
implementations.

Note, HCI Mesh extensions not supported in ULL/LLL arch yet,
hence disabled. Also, only a partial implementation present
in the old architecture.

Relates to #12860.

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