Calculating Euclidean distance and performing unit-testing
Since the tests are testing the same thing (calculate the distance between two nodes) and the only difference is the test data, I say refactor it so that you have one test that tests the same logic with different data. When you have tests that are there for different purposes (like validation checking, distance calculation, not calling createLink, etc) you will have code duplication.

There should be separate tests for addNode and separate tests for validation checks (what happens when you try to create a link when there are no nodes added). Also there should be different variations of the amount of nodes.

Setup is called before every testmethod, so if you always create the same SpatialGraph you can do that there.