Bluetooth: testlib: Add `BT_TESTLIB_ADDR_LE_RANDOM_C0_00_00_00_00_`

This is shorthand for random static addresses. It's similar to
`bt_addr_le_from_str`, but is a macro that results in an object literal,
making it more versatile and less verbose.

This macro only gives access to the first 255 random static addresses,
but this ought to be enough addresses for testing.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>