test: add single-byte encoding tests

Author: ChALkeR

test/fixtures/encoding/README.md

@@ -0,0 +1,4 @@
+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>