tests: Fix fake-pinentry for Windows.

* tests/openpgp/fake-pinentry.c (main): Override PINENTRY_USER_DATA,
by the option.

In the Assuan implementation for Windows, spawn function doesn't call
the atfork callback. Thus, the environment variable is not updated by
gpg-agent when it spawns pinentry. Reliable way is the interaction
to override the option.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>