Fix Draft 3 ecmascript-regex look-behind test (#906)

Lookbehind assertions were added to ECMA-262 in ES2018 and are accepted
by every modern JavaScript engine, so "(?<=foo)bar" is in fact a valid
ECMA 262 regex. The existing test asserting otherwise no longer reflects
the specification.

I know we discourage work on older drafts but bear with me! I need good
Draft 3 support for my auto-upgrades :)

Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>
Co-authored-by: Juan Cruz Viotti <jviotti@sourcemeta.com>

Author: jviotti

tests/draft3/optional/format/ecmascript-regex.json

@@ -9,8 +9,13 @@
                 "valid": true
             },
             {
-                "description": "ECMA 262 has no support for lookbehind",
+                "description": "ECMA 262 supports lookbehind since ES2018",
                 "data": "(?<=foo)bar",
+                "valid": true
+            },
+            {
+                "description": "ECMA 262 does not support Python-style named groups",
+                "data": "(?P<name>x)",
                 "valid": false
             }
         ]