tests/drbgtest: use new RAND_DRBG callback_data API instead of ex_data

It took me a little while to realize why the test_rand_drbg_reseed test
kept crashing after replacing the RAND_DRBG_{gs}et_ex_data() calls by
RAND_DRBG_{gs}et_callback_data().

The reason was that the ex_data API prohibits modifying the callbacks
or callback data of chained DRBGs and returned an error which was
ignored by the `test_rand_drbg_reseed` test, for good reasons.

The `test_rand_drbg_reseed` test is special in this respect, because
it needs to install callbacks for all DRBGs, in order to intercept
and count the reseeding events.

Since the drbgtest module has access to the internal structures of
the DRBG anyway, the problem could be solved by accessing the members
directly. I added a warning comment in hook_drbg().

[extended tests]

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/10950)

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