More tests for edge_info argument.

[ashenvi10]

Add tests to ensure that the edge_info argument for figure creation is working fine.

Tests have been added to ensure that it shows the right warning message when the argument is inadmissible (i,e. it is a non-existent attribute) but not for when the argument is admissible. For event trees, this only means 'count' and so, no additional tests needed. Additional tests needed for staged trees and cegs as the argument can be set to any of the following: 'prior', 'count', 'posterior', 'probability'.