random: Clarify the semantics of TEST_RANDOM_GENERATOR

The `TEST_RANDOM_GENERATOR` Kconfig symbol indicates that a non-random
number generator (i.e. a random number generator that is not truly
random) is _allowed_ to be used for testing purposes, and not
necessarily that a non-random number generator is _selected_ -- that is
indicated by the `RNG_GENERATOR_CHOICE` choices that depend on
`TEST_RANDOM_GENERATOR` (i.e. only `TIMER_RANDOM_GENERATOR` as of now).

This commit updates the `TEST_RANDOM_GENERATOR` Kconfig symbol
description to reflect and clarify that.

It also removes the `TEST_RANDOM_GENERATOR`'s dependency on
`ENTROPY_HAS_DRIVER` because the act of _allowing_ a non-random number
generator to be used does not depend on the availability of an entropy
driver -- when an entropy driver is available, by default, the
`ENTROPY_DEVICE_RANDOM_GENERATOR` will be selected; otherwise,
`TIMER_RANDOM_GENERATOR`, a non-random generator, will be selected.

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