dhtest.c: Add test of DH_check() with q = p + 1

dhtest.c: Add test of DH_check() with q = p + 1

This must fail with DH_CHECK_INVALID_Q_VALUE and
with DH_CHECK_Q_NOT_PRIME unset.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/21550)

(cherry picked from commit ad5d35572695d7b5748b2bd4fb1afaa189b29e28)