test/drbgtest: improve the reseed after fork test

Issue #12377 demonstrated that it is not sufficient to verify that
after a fork a reseeding is triggered in the child. This commit
enhances the test by collecting the output of the public and private
drbg for the parent and all children and checking for duplicates.
In case of duplicates, it prints an error message and displays a
sorted output.

The analysis of #12377 (see [1]) showed that due to an error in the
resetting of the AES-CTR (issue #12405, fixed by #12413), it could
happen that only the first n bytes (n=1,...15) of the children's
random output were identical.

This test is optimized to detect this issue by only comparing the first
byte of the sampled data (i.e., the first 'column' of the output).
The number of samples is chosen high enough to keep the chance of false
positives low. The test is executed sixteen times, each time advancing
the internal counter by requesting a single extra byte of random data.

Another, more general test splits the entire sampled random data into
two-byte chunks and counts their collisions. If a certain threshold
is exceeded, it reports an error.

[1] https://github.com/openssl/openssl/issues/12377#issuecomment-656207334

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/12407)

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