fix: add a test to check for circle ring closure

[llamington]

Adds a test to satisfy #289

[Fil]

Upgrading the test is not enough; we'll want to add a test for the case that breaks, and fix the generator:

d3-geo/src/circle.js

Line 20 in 8c53a90

for (var point, t = t0; direction > 0 ? t > t1 : t < t1; t -= step) {

(this should probably store the starting point and repeat it at the end).