Fix hanging test262 bug by setting the length on String objects (#728)

Co-authored-by: Halid Odat <halidodat@gmail.com>

boa/src/exec/tests.rs

@@ -33,6 +33,15 @@ fn property_accessor_member_expression_bracket_notation_on_string_literal() {
     assert_eq!(&exec(scenario), "\"function\"");
 }
 
+#[test]
+fn length_correct_value_on_string_literal() {
+    let scenario = r#"
+    'hello'.length;
+    "#;
+
+    assert_eq!(&exec(scenario), "5");
+}
+
 #[test]
 fn property_accessor_member_expression_dot_notation_on_function() {
     let scenario = r#"

boa/src/value/mod.rs

@@ -713,10 +713,10 @@ impl Value {
                     .expect("String was not initialized")
                     .get_field(PROTOTYPE);
 
-                Ok(GcObject::new(Object::with_prototype(
-                    proto,
-                    ObjectData::String(string.clone()),
-                )))
+                let mut obj = Object::with_prototype(proto, ObjectData::String(string.clone()));
+                // Make sure the correct length is set on our new string object
+                obj.set("length".into(), string.chars().count().into());
+                Ok(GcObject::new(obj))
             }
             Value::Symbol(ref symbol) => {
                 let proto = ctx