Add corner case tests for ECMA 262 regexes

Add a very bizarre construct which only ECMA 262 allows in regexes. Check that
lookbehinds are _not_ supported.

Author: fge

tests/draft3/optional/jsregex.json

@@ -0,0 +1,18 @@
+[
+    {
+        "description": "ECMA 262 regex dialect recognition",
+        "schema": { "format": "pattern" },
+        "tests": [
+            {
+                "description": "[^] is a valid regex",
+                "data": "[^]",
+                "valid": true
+            },
+            {
+                "description": "ECMA 262 has no support for lookbehind",
+                "data": "(?<=foo)bar",
+                "valid": false
+            }
+        ]
+    }
+]