Improve coverage of MapIteratorPrototype

test/built-ins/MapIteratorPrototype/Symbol.toStringTag.js

...type/next/does-not-have-mapiterator-internal-slots-map.js

...rototype/next/does-not-have-mapiterator-internal-slots.js

.../built-ins/MapIteratorPrototype/next/iteration-mutable.js

test/built-ins/MapIteratorPrototype/next/iteration.js

test/built-ins/MapIteratorPrototype/next/no-map-data.js

...apIteratorPrototype/next/this-not-object-throw-entries.js

...s/MapIteratorPrototype/next/this-not-object-throw-keys.js

...rototype/next/this-not-object-throw-prototype-iterator.js

...MapIteratorPrototype/next/this-not-object-throw-values.js