Test stoex editing functionality

Description

This test checks the functionality of the stoex xtext editor.

New stoex xtext implementation

	Test Step	Expected Result
1	Create an element that contains a RandomVariable, e.g., in a SEFF SetVariableAction	The element is created but if there is no standard value available, its value is null. An error marker is then shown besides it.
2	Open the StoEx xtext editor: Either press F2 or do a slow double-click.	The inline xtext editor is opened, content assist is available
3	Edit StoEx: Enter an invalid expression	When pressing "Enter", the editor does not close but forces focus. The background of the editor is red, errors are displayed through hovering over the "info" label at the left of the editor.
4	Edit StoEx: Stop editing through pressing "Esc"	The editor is closed and the value of the RandomVariable is the same as before.
5	Edit StoEx: Enter a valid expression, e.g., "true"	When pressing "Enter", the editor closes and the value of the RandomVariable is updated. The error marker is not shown anymore.