Add End-to-End-Tests for multilevel accordions

[schaertim]

The tests should make sure the opening/closing logic of accordion items works as expected in multilevel accordions, especially for the case where multiple open items are not allowed.

Possible testcases could be:

• Open top level item
• Open lower level item
• Open another lower level item

=> First lower level item should close and the second lower level item should be open

• Wrap lower level accordion item inside div
• Repeat test described above

=> Same behavior as above