Bluetooth: Add selftest for ECDH key generation

Since the ECDH key generation takes a different path, it needs to be
tested as well. For this generate the public debug key from the private
debug key and compare both.

This also moves the seeding of the private key into the SMP calling code
to allow for easier re-use of the ECDH key generation helper.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>