Currently, tests start with an empty root node. FMUI would benefit from the ability to read (exported) YAMLs into the root so tests can be done over existing data.

It would depend on the existence of a session in a test run and would import data within it.

Acceptance criteria

• Read YAMLs into the root node so tests can work with the existing data. 
• Consider making it a JUnit 5 extension, but still, have it available in JUnit 4
• Remove YAMLImporter class from magnolia-ui-framework-jcr package and replace its usage with the new mechanism.