add flatten/flatMap to features.txt and mark appropriate tests

test/built-ins/Array/prototype/flatten/positive-infinity.js

@@ -5,6 +5,7 @@ esid: sec-array.prototype.flatten
 description: >
     if the argument is a positive infinity, the depthNum is max depth of the array
 includes: [compareArray.js]
+features: [Array.prototype.flatten]
 ---*/
 
 var a = [1, [2, [3, [4]]]]

test/built-ins/Array/prototype/flatten/prop-desc.js

@@ -7,6 +7,7 @@ description: Property type and descriptor.
 info: >
   17 ECMAScript Standard Built-in Objects
 includes: [propertyHelper.js]
+features: [Array.prototype.flatten]
 ---*/
 
 assert.sameValue(

test/built-ins/Array/prototype/flatten/symbol-object-create-null-depth-throws.js

@@ -4,6 +4,7 @@
 esid: sec-array.prototype.flatten
 description: >
     if the argument is a Symbol or Object null, it throws exception
+features: [Array.prototype.flatten]
 ---*/
 
 assert.throws(TypeError, function() {