test/drbgtest.c: call OPENSSL_thread_stop() explicitly

The manual says this in its notes:

    ... and therefore applications using static linking should also call
    OPENSSL_thread_stop() on each thread. ...

Fixes #8171

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/8173)