Bluetooth: Host: Add legacy pairing test config

Adds the `CONFIG_BT_SMP_LEGACY_PAIR_ONLY` Kconfig option to force
devices to use legacy pairing. This has a dependency on
`CONFIG_BT_TESTING` as it is only intended for testing purposes, and use
of legacy pairing is discouraged.

Signed-off-by: Håvard Reierstad <haavard.reierstad@nordicsemi.no>