patch 9.1.0776: test_strftime may fail because of missing TZ data

patch 9.1.0776: test_strftime may fail because of missing TZ data

Problem:  test_strftime may fail because of missing TZ data
Solution: Use GMT offsets to validate timezone differences (James McCoy)

Some systems only provide timezones that follow the geographical region
naming (e.g. America/New_York) by default and require an additional
install for other names (like EST).

The GMT+<offset> format must always be honored, so use that to generate
distinct data for the test.

closes: #15848

Signed-off-by: James McCoy <jamessan@jamessan.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>