tests: Use 233 for invalid value of FD.

* tests/openpgp/issue2941.scm: Use 233.

Forward port from 2.2 branch of:
43722438a826e1a162723a23452018ccf1b640ec

On Windows machine (emulated by Wine), 23 may be valid value for
handle.

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