The test was expecting timestamps on day 1 in 1970 to start at 86400 but
they actually started at 0. This worked without `SOURCE_DATE_EPOCH=1`
because after 1972, leap days compensated the offset.
Fixes: #17762
The new `%s` format specifier is substituted by file()/string()
`TIMESTAMP` sub-commands with the number of seconds since unix-epoch
(1970-01-01 00:00:00 UTC).
Co-Author: Nils Gladitz <nilsgladitz@gmail.com>