random: Fix non-random number generator warning condition

The `TEST_RANDOM_GENERATOR` symbol in itself does not imply that a
non-random number generator is selected -- that is indicated by the
`RNG_GENERATOR_CHOICE` choices, of which only `TIMER_RANDOM_GENERATOR`
is a non-random generator.

This commit updates the Random subsystem to print the "non-random
number generator usage warning" when `TIMER_RANDOM_GENERATOR` is
selected, as opposed to when `TEST_RANDOM_GENERATOR` is selected.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>