Add a test for signing with certificates in Windows store

Add a test for signing with certificates in Windows store

- For each sample certificate/key pair imported into the store,
  load the key into xkey-provider and sign a test message.
  As the key is "provided", signing will use appropriate
  backend (Windows CNG in this case).

  The signature is then verified using OpenSSL.

Change-Id: I520b34ba51e8c6d0247a82edc52bde181ab5a717
Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20230315013516.1256700-5-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26416.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>