Tests: Bluetooth: Build Host with different configurations

The new `host_config_variants` goal is to be able to test specific set
of configurations when building the Host without the need of having a
specific application for it.

The first test check that the Host build correctly with
`CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY` enabled.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>