Fix the drbgtest with randomized ordering

In drbgtest, test_set_defaults changes the default DRBGs. This works fine
when tests are run in the normal order. However if
OPENSSL_TEST_RAND_ORDER is defined then it may fail (dependent on the
ordering). This environment variable is defined for one of the Travis
tests, so this issue was causing intermittent travis test failures.

[extended tests]

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7338)