MDL-56609 phpunit: Fix bogus expectation on DST changes day
Current test works ok every day of the year but the day of
DST begin changes (last Sunday of March currently).
That day, 01:00 Europe/London simply doesn't exist, because
the time is moved forward by 60 minutes.
Hence the expectation for the next cron execution is not 1 "normal"
day (24h) but 1 less hour.
Note this only happens for the current test on DST begins, the
opposite change (DST ends), happening the last Sunday of October
is not affected, because, at all effects, 01:00 is a perfectly normal
and existing hour.