add escaped substitution test

test/built-ins/RegExp/named-groups/string-replace-escaped.js

+// Copyright 2017 Aleksey Shvayka. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: Do not replace $<> preceded with $
+esid: sec-getsubstitution
+features: [regexp-named-groups]
+info: >
+  Runtime Semantics: GetSubstitution( matched, str, position, captures, namedCaptures, replacement )
+
+  12. These $ replacements are done left-to-right, and, once such a replacement is performed,
+  the new replacement text is not subject to further replacements.
+
+  Table: Replacement Text Symbol Substitutions
+
+  Unicode Characters: $$
+  Replacement text: $
+---*/
+
+let source = "(?<fst>.)";
+for (let flags of ["", "u"]) {
+  let re = new RegExp(source, flags);
+  assert.sameValue("$<fst>bc", "abc".replace(re, "$$<fst>"));
+}