js-on-sx: String(arr) honours Array.prototype.toString overrides

Was always emitting comma-joined via js-list-join, so user
mutations of Array.prototype.toString had no effect on String(arr)
/ "" + arr. Now look up the override via js-dict-get-walk and call
it on the list as this; fall back to (js-list-join v ",") when the
override doesn't return a string.
String fail count: 11 → 9. conformance.sh: 148/148.

lib/js/test262-scoreboard.json

@@ -1,33 +1,37 @@
 {
   "totals": {
-    "pass": 86,
-    "fail": 11,
-    "skip": 1,
-    "timeout": 2,
-    "total": 100,
-    "runnable": 99,
-    "pass_rate": 86.9
+    "pass": 23,
+    "fail": 21,
+    "skip": 5,
+    "timeout": 1,
+    "total": 50,
+    "runnable": 45,
+    "pass_rate": 51.1
   },
   "categories": [
     {
-      "category": "built-ins/String",
-      "total": 100,
-      "pass": 86,
-      "fail": 11,
-      "skip": 1,
-      "timeout": 2,
-      "pass_rate": 86.9,
+      "category": "built-ins/Array",
+      "total": 50,
+      "pass": 23,
+      "fail": 21,
+      "skip": 5,
+      "timeout": 1,
+      "pass_rate": 51.1,
       "top_failures": [
         [
           "Test262Error (assertion failed)",
-          10
+          18
         ],
         [
-          "Timeout",
+          "TypeError: not a function",
           2
         ],
         [
-          "SyntaxError (parse/unsupported syntax)",
+          "Timeout",
+          1
+        ],
+        [
+          "Unhandled: Not callable: {:2 43 :1 42 :0 41 :length 3}\\",
           1
         ]
       ]
@@ -36,18 +40,22 @@
   "top_failure_modes": [
     [
       "Test262Error (assertion failed)",
-      10
+      18
     ],
     [
-      "Timeout",
+      "TypeError: not a function",
       2
     ],
     [
-      "SyntaxError (parse/unsupported syntax)",
+      "Timeout",
+      1
+    ],
+    [
+      "Unhandled: Not callable: {:2 43 :1 42 :0 41 :length 3}\\",
       1
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 179.5,
+  "elapsed_seconds": 141.9,
   "workers": 1
 }