tests: Simplify fake-pinentry to use the option only.

* tests/openpgp/fake-pinentry.c (parse_pinentry_user_data): New.
(main): Don't use PINENTRY_USER_DATA env var.

Since environment variable is unreliable, use the option only.

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