Fork tree prune assumptions removal v2 (#13327)

* Removed assumptions about ancestry from fork tree prune method

* Tests improvement

* Fork tree prune refactory

* Code refactory

* Correctly handle borderline, but legit, case

* Apply suggestions from code review

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

* Removed duplicated test

---------

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

substrate/client/consensus/epochs/src/lib.rs

@@ -1039,7 +1039,7 @@ mod tests {
 
 		let mut nodes: Vec<_> = epoch_changes.tree().iter().map(|(h, _, _)| h).collect();
 		nodes.sort();
-		assert_eq!(nodes, vec![b"A", b"B", b"C", b"E", b"F", b"G"]);
+		assert_eq!(nodes, vec![b"A", b"B", b"C", b"F", b"G"]);
 
 		// Finalize block y @ number 35, slot 330
 		// This should prune all nodes imported by blocks with a number < 35 that are not
@@ -1050,7 +1050,7 @@ mod tests {
 
 		let mut nodes: Vec<_> = epoch_changes.tree().iter().map(|(h, _, _)| h).collect();
 		nodes.sort();
-		assert_eq!(nodes, vec![b"B", b"C", b"F", b"G"]);
+		assert_eq!(nodes, vec![b"B", b"C", b"G"]);
 	}
 
 	#[test]