test-rand: return failure on not enough data, allow parent

test-rand: return failure on not enough data, allow parent

The test-rand RNG was returning success when it had some but insufficient data.
Now, it returns failure and doesn't advance the data pointer.

The test-rand RNG was failing when a parent was specified.  This case is now
ignored.

Fixes #16785

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16905)