move all plugin tests out of babel-core and into their appropriate folders

packages/babel-plugin-transform-es2015-function-name/test/fixtures/function-name/assignment/actual.js

@@ -0,0 +1,9 @@
+var i = function () {
+  i = 5;
+};
+
+var j = function () {
+  ({ j } = 5);
+  ({ y: j } = 5);
+  ;
+};

packages/babel-plugin-transform-es2015-function-name/test/fixtures/function-name/assignment/expected.js

@@ -0,0 +1,9 @@
+var _i = function i() {
+  _i = 5;
+};
+
+var _j = function j() {
+  ({ j: _j } = 5);
+  ({ y: _j } = 5);
+  ;
+};