test: add WHATWG Encoding indexes.json to fixtures

Author: ChALkeR

tests/encoding/fixtures/README.md

@@ -1,6 +1,7 @@
 1. <https://encoding.spec.whatwg.org/>
 2. <https://encoding.spec.whatwg.org/encodings.json>
-3. <https://encoding.spec.whatwg.org/#indexes>
-4. <https://encoding.spec.whatwg.org/#legacy-single-byte-encodings>
-5. <https://unicode.org/Public/MAPPINGS/ISO8859/>
-6. <https://unicode.org/Public/MAPPINGS/VENDORS/>
+3. <https://encoding.spec.whatwg.org/indexes.json>
+4. <https://encoding.spec.whatwg.org/#indexes>
+5. <https://encoding.spec.whatwg.org/#legacy-single-byte-encodings>
+6. <https://unicode.org/Public/MAPPINGS/ISO8859/>
+7. <https://unicode.org/Public/MAPPINGS/VENDORS/>