Add a test to check that RAND_bytes_ex() works with a child lib ctx

Add a test to check that RAND_bytes_ex() works with a child lib ctx

Previously, when locks were held while calling a provider init function,
then RAND_bytes_ex() would fail if called from the init function and
used in conjunction with a child lib ctx. We add an explicit test of that.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15854)