Add note about third-party test fixture code

Author: isaacs

test/fixtures/third-party.md

@@ -0,0 +1,25 @@
+"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:
+
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