tests: Fix a test which specifies expiration date.

* tests/openpgp/quick-key-manipulation.scm: Fix expiration time
comparison.

This is a bug fix for Amelia Earhart who is probably in UTC-12.

When expiration date is specified, GnuPG interprets it as noon of the
date in local time.

Before this fix, the test compared the value by 2145916800 which is
2038-01-01 00:00:00 in UTC with allowance of 1 day. When the test
was ran in UTC-12 timezone, it failed because of noon in the timezone
is midnight of the next day in UTC.

• GnuPG-bug-id: T3393
• Reported-by: Daniel Kahn Gillmor
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>