enable backref test

[pjfanning]

relates to #4642 and #1516

useful to have the working test enabled for regression purposes

[cowtowncoder]

As per other comments, I'm "meh" on this.

But if passing tests to be moved, they are to go next to existing back-reference tests:

src/test/java/com/fasterxml/jackson/databind/struct/

(like src/test/java/com/fasterxml/jackson/databind/struct/BackReference1878Test.java)