Fix: drbgtest fails when tests are executed in random order

[extended tests]

The test_rand_reseed assumed that the global DRBGs were not used
previously. This assumption is false when the tests are executed
in random order (OPENSSL_TEST_RAND_ORDER). So we uninstantiate
them first and add a test for the first instantiation.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5680)