LPS-45858 Modify test to get the modified and last merge time date of the updated layout.

This causes the test to fail and reveals an issue: the last merge date of layout A is updated when another layout B is reset, even if layout A hasn't been reset.
LPS-45858 Consider the first propagation case

In this scenario, the last merge time is 0 and therefore the modified date is greater than it, but the layout must be merged.
LPS-45858 Skip portlet preferences import if the page has been modified in the site

The last merge time of the layout is not updated if it hasn't been merged because it has been modified in the site.
The last merge time of the layout set is not updated unless all layouts have been merged.
LPS-45858 Reuse existing logic
LPS-45858 Add test util logic to set portlet preferences
LPS-45858 Add new reset test case for portlet preferences
LPS-45858 SF