[3.5.x] Fix strftime unit test.

The test tests/acceptance/01_vars/02_functions/strftime.cf would fail
builds on Sundays. The policy specified Sunday as 7 but strftime specifies
Sunday as 0.

From strftime(3):

     %w    is replaced by the weekday (Sunday as the first day of the week)
           as a decimal number (0-6).
(cherry picked from commit 4d6b69fbd2f421725b7f1308b15190c7b933c7fe)