Ensure valid input to DepthSegment constructor in unit test

[FObermaier]

Whilst struggling with an NTS issue related to BufferOp I noticed that the input for the DepthSegmentTest is not valid according to the assumptions made in the DepthSegment constructor:

jts/modules/core/src/main/java/org/locationtech/jts/operation/buffer/SubgraphDepthLocater.java

Lines 164 to 170 in 8e2f394

	public DepthSegment(LineSegment seg, int depth)
	{
	// input seg is assumed to be normalized
	upwardSeg = new LineSegment(seg);
	//upwardSeg.normalize();
	this.leftDepth = depth;
	}

This mentioned issue does not affect JTS.