Fix some potential race conditions in tests.

Unless .html("") is called on d3.select("body"), elements appended by
another test could interfere with data binding.

See #329.

test/core/transition-test-attrTween.js

@@ -11,7 +11,7 @@ module.exports = {
         tt = [],
         vv = [];
 
-    var s = d3.select("body").append("div").selectAll("div")
+    var s = d3.select("body").html("").append("div").selectAll("div")
         .data(["red", "green"])
       .enter().append("div")
         .attr("color", function(d) { return d3.rgb(d)+""; });

test/core/transition-test-each.js

@@ -7,7 +7,7 @@ module.exports = {
   "start": {
     topic: function() {
       var cb = this.callback,
-          div = d3.select("body").selectAll().data(["foo", "bar"]).enter().append("div").attr("class", String),
+          div = d3.select("body").html("").selectAll().data(["foo", "bar"]).enter().append("div").attr("class", String),
           transition = div.transition().delay(150),
           then = Date.now(),
           n = 0,
@@ -79,7 +79,7 @@ module.exports = {
   "end": {
     topic: function() {
       var cb = this.callback,
-          div = d3.select("body").selectAll().data(["foo", "bar"]).enter().append("div").attr("class", String),
+          div = d3.select("body").html("").selectAll().data(["foo", "bar"]).enter().append("div").attr("class", String),
           transition = div.transition().duration(150),
           then = Date.now(),
           n = 0,

test/core/transition-test-styleTween.js

@@ -12,7 +12,7 @@ module.exports = {
         vv = [],
         fails = 0;
 
-    var s = d3.select("body").append("div").selectAll("div")
+    var s = d3.select("body").html("").append("div").selectAll("div")
         .data(["red", "green"])
       .enter().append("div")
         .style("background-color", function(d) { return d3.rgb(d)+""; });