[FIX] web: calendar's test to create an event with timezone

In `calendar_tests.js`, the tests about creating an event with timezone in
week mode only triggers 7 asserts but they were design to trigger 8 of
them.

After analysis, it looks like in the commit odoo/odoo@369c20a99bd25172f72f4efde870172a4729aadc
the FullCalendar's UI was redesigned and some CSS rules might have add
some margin preventing a MouseEvent from being triggered correctly,
resulting in misadapted tests.

This commit restores the original behavior by adding some pixels to
these offsets, allowing the events to be correctly dispatched and
adapting again the tests.

Task ID: 2092550