Fix new test for clocktables
* testing/lisp/test-org-clock.el (test-org-clock/clocktable): The last test
for clocktables introduced in commit 6642177 did not work if the test was
run before 15:00 due to the end time being specified as "<now>" (which
includes the current time of day and not just the date). The obvious
"<today>" does also not work since it means 0:00 of the current day. The
correct specification to use is "<tomorrow>", which is 0:00 the following
day or equivalently the end of today, 24:00.