random: `sys_csrand_get` backend for `TEST_RANDOM_GENERATOR`

When non-random number generation is allowed via
`TEST_RANDOM_GENERATOR`, enable an implementation for `sys_csrand_get`
that stubs out to `sys_rand_get`. This enables libraries that request
CS random numbers to be tested in CI, even if the results are not CS in
that context.

The documentation for `TEST_RANDOM_GENERATOR` is explicit enough about
the dangers of enabling this in production.

Signed-off-by: Jordan Yates <jordan@embeint.com>