abstract out test runner into a module, move traceur and esnext tests to babel-preset-es2015, clean up and make existing tests more consistent

packages/babel-cli/test/fixtures/babel-external-helpers/--output-type var/stdout.txt

@@ -1 +1,2 @@
 var babelHelpers = {};
+babelHelpers;

packages/babel-cli/test/index.js

@@ -1,7 +1,7 @@
 if (process.env.running_under_istanbul) return;
 
 var readdir        = require("fs-readdir-recursive");
-var helper         = require("mocha-fixtures");
+var helper         = require("babel-helper-fixtures");
 var assert         = require("assert");
 var rimraf         = require("rimraf");
 var outputFileSync = require("output-file-sync");