LPS-33307 Convert content to XML when updating

This is already done when adding.
LPS-33307 Move common layout logic to LayoutTestUtil
LPS-33307 Use JournalTestUtil for operations with journal contents
LPS-33307 Rename tests

LayoutExportImportTest and PortletExportImportTest are actually test cases for LayoutSetPrototype propagation. LayoutExporter, LayoutImporter, PortletExporter and PortletImporter classes are implicitly tested here, but not exclusively. Other classes such as SitesUtil or LayoutLocalServiceVirtualLayoutsAdvice are also involved in the propagation of LayoutSetPrototypes.

PortletExportImportTest will be merged with LayoutSetPrototypePropagationTest in next commit.
LPS-33307 Refactor prototype propagation tests

Move tests from PortletExportImportTest to LayoutSetPrototypePropagationTest and prepare BasePrototypePropagationTestCase for other prototypes.
LPS-33307 Add tests to validate if a layout is sortable, updateable and deletable
LPS-33102 Add tests for customizations in a page created from a site template
LPS-33307 Add tests for Layout Prototype