MergeHistory: Fix flaky test due to relative timestamp

MergeHistory: Fix flaky test due to relative timestamp

In PHP, `strtotime('tomorrow')` evaluates to the first midnight
after the current time.

Which means on Aug 14 at 23:59:59, strtotime('tomorrow') refers
to a timestamp merely 1 second in the future (Aug 15 00:00:00).

This causes the test to fail because this timestamp is computed
in a data provider, before the test itself starts. In order to
pass, it needs to be far enough in the future that it will also
be after the point in time where the test creates the page and
revisions in question.

Fix this by:
* ... moving it into the test itself, where even plain time()
      should probably suffice, but that's for another time.

* ... using +24*3600 instead of strtotime().

Bug: T201976
Change-Id: Ice5d54af4fb7cffa8db9b657796ba6ebecdaffa0