test/drbgtest.c: Fix error check test

The condition in test_error_checks() was inverted, so it succeeded
as long as error_check() failed. Incidently, error_check() contained
several bugs that assured it always failed, thus giving overall drbg
test success.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/11195)
