Improve RegExp.prototype.flags coverage (#1149)

test/built-ins/RegExp/prototype/flags/coercion.js

test/built-ins/RegExp/prototype/flags/length.js

test/built-ins/RegExp/prototype/flags/name.js

test/built-ins/RegExp/prototype/flags/order-gets.js

test/built-ins/RegExp/prototype/flags/prop-desc.js

test/built-ins/RegExp/prototype/flags/rethrow.js

test/built-ins/RegExp/prototype/flags/s.js

test/built-ins/RegExp/prototype/flags/this-val-non-obj.js

...t-ins/RegExp/prototype/flags/this-val-regexp-prototype.js

test/built-ins/RegExp/prototype/flags/this-val-regexp.js

test/built-ins/RegExp/prototype/flags/u-attr-err.js

test/built-ins/RegExp/prototype/flags/u-coercion.js

test/built-ins/RegExp/prototype/flags/u.js

test/built-ins/RegExp/prototype/flags/y-attr-err.js

test/built-ins/RegExp/prototype/flags/y.js