Add must-fail tests for padding byte in non-terminal position

From https://httpwg.org/specs/rfc9651.html#parse-binary:

> This specification does not relax the requirements in Sections 3.1 and 3.3 of
RFC4648; therefore, parsers MUST fail on characters outside the base64
alphabet and on line feeds in encoded data.

binary.json

@@ -15,6 +15,18 @@
             {"__type": "binary", "value": ""},
             []]
     },
+    {
+        "name": "padding at beginning",
+        "raw": [":=aGVsbG8=:"],
+        "header_type": "item",
+        "must_fail": true
+    },
+    {
+        "name": "padding in middle",
+        "raw": [":a=GVsbG8=:"],
+        "header_type": "item",
+        "must_fail": true
+    },
     {
         "name": "bad padding",
         "raw": [":aGVsbG8:"],