[umbrella][scheduler] Add more tests

[rita-codes]

Important

• Review all the tests inside the styled version to check if some behaviors are only covered/tested there; if we find something, add a test in the headless version.

x-scheduler

// TODO after review

x-scheduler-headless

[calendar-grid]:

• [ ] useDayCellDropTarget (inside day-cell)
• [ ] useTimeDropTarget (inside time-column)
• [ ] useCalendarGridPlaceholderInDay (inside use-placeholder-in-day)
• [ ] useCalendarGridPlaceholderInRange (inside use-placeholder-in-range)

[event-calendar-selectors]:

• [ ] placeholderInDayCell
• [ ] placeholderInTimeRange

[scheduler-selectors]:

• [x] defaultPresetKey (schedulerRecurringEventSelectors)
• [x] processedResourceFlatList (schedulerResourceSelectors)
• [ ] isStartedOrRender (schedulerOccurrenceSelectors)

[timeline]:

• [ ] useEventRowDropTarget (event-row)
• [ ] useTimelinePlaceholderInRange (use-placeholder-in-range)

[timeline-selectors]:

• [ ] timelineOccurrencePlaceholderSelectors

[use-timeline]:

• [ ] setView (timelineStore)

[utils]:

• [ ] useDraggableEvent
• [ ] useDragPreview
• [ ] useDropTarget
• [ ] useElementPositionInCollection
• [ ] useEventResizeHandler

[utils/recurring-events]:

• [x] computeMonthlyOrdinal