Add note about third-party test fixture code

test/fixtures/third-party.md

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+"scoped-underscore-1.3.1.tgz" includes content from the Underscore package,
+including code adapted from ES 5.1 section 15.12.3, abstract operation
+`JO`, used under the following license:
+
+Copyright (c) Ecma International 2010 
+
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+copyright notice and this section are included on all such copies and
+derivative works. However, this document itself may not be modified in any
+way, including by removing the copyright notice or references to Ecma
+International, except as needed for the purpose of developing any document
+or deliverable produced by Ecma International.
+
+The limited permissions are granted through the standardization phase and
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+
+This document and the information contained herein is provided on an "AS
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