tests: add Kconfig file to bluetooth ctrl_data_length_update testcases

With Kconfig support in unittest the bluetooth ctrl_data_length_update
test fails because the symbols being configured are promptless or
disabled because of dependencies.

Add a Kconfig file to make needed symbols visible in unittest mode.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>