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

features.txt

test/built-ins/Array/prototype/flatMap/array-like-objects.js

...lt-ins/Array/prototype/flatMap/bound-function-argument.js

test/built-ins/Array/prototype/flatMap/depth-always-one.js

test/built-ins/Array/prototype/flatMap/length.js

test/built-ins/Array/prototype/flatMap/name.js

...s/Array/prototype/flatMap/non-callable-argument-throws.js

...ilt-ins/Array/prototype/flatMap/non-object-ctor-throws.js

...ns/Array/prototype/flatMap/null-undefined-input-throws.js

test/built-ins/Array/prototype/flatMap/thisArg-argument.js

test/built-ins/Array/prototype/flatten/array-like-objects.js

.../built-ins/Array/prototype/flatten/bound-function-call.js

...built-ins/Array/prototype/flatten/empty-array-elements.js

...uilt-ins/Array/prototype/flatten/empty-object-elements.js

test/built-ins/Array/prototype/flatten/length.js

test/built-ins/Array/prototype/flatten/name.js

...y/prototype/flatten/non-numeric-depth-should-not-throw.js

...ilt-ins/Array/prototype/flatten/non-object-ctor-throws.js

...lt-ins/Array/prototype/flatten/null-undefined-elements.js

...ns/Array/prototype/flatten/null-undefined-input-throws.js