fips selftest: avoid relying on a real RNG for self tests

fips selftest: avoid relying on a real RNG for self tests

Rather than instantiate the private and primary DRBGs during the
selftest, instead use a test RNG.  This leaves the DRBG setup
pristine and permits later replacement of the seed source despite
the very early running power up self tests.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21964)