Add test for d3.event re-export on Node. Related d3#3055.

Author: mbostock

test/d3-test.js

@@ -1,12 +1,24 @@
 var tape = require("tape"),
     d3 = require("../"),
+    d3Selection = require("d3-selection"),
     testExports = require("./test-exports");
 
 tape("version matches package.json", function(test) {
   test.equal(d3.version, require("../package.json").version);
   test.end();
 });
 
+tape("d3.event is a getter for d3Selection.event", function(test) {
+  test.equal(d3.event, null);
+  try {
+    d3Selection.event = 42;
+    test.equal(d3.event, 42);
+  } finally {
+    d3Selection.event = null;
+  }
+  test.end();
+});
+
 for (var dependency in require("../package.json").dependencies) {
   testExports(dependency);
 }