Test case for minimum distance

These test cases pertain to repeated labels and minimum distance. The behavior for line placement in text symbolizer previously was to only check the distance with repeated text. The current behavior only checks the distance from all other labels. See mapnik/mapnik#2227.

This is simply a test case to show how minimum distance current effects labels, blocking labels within a certain distance of all other labels.

I think an ideal solution that provides more flexibility would be to add a separate repeat-distance attribute to check for repeated labels. I have implemented a solution for that on another branch and will submit a PR for that shortly. I kept these test cases separate, however, in case a different solution is desired.