Panic by default in serde_test is_human_readable

The serde_test Serializer and Deserializer panic in is_human_readable unless the
readableness has been set explicitly through one of the hidden functions. This
is to force types that have distinct readable/compact representations to be
tested explicitly in one or the other, rather than with a plain assert_tokens
which arbitrarily picks one.

We need to follow up by designing a better API in serde_test to expose this
publicly. For now serde_test cannot be used to test types that rely on
is_human_readable. (The hidden functions are meant for our test suite only.)

test_suite/tests/test_roundtrip.rs

@@ -19,7 +19,7 @@ fn ip_addr_roundtrip() {
             seq b"1234".iter().map(|&b| Token::U8(b)),
             Token::TupleEnd,
         ],
-        false,
+        Some(false),
     );
 }
 
@@ -40,6 +40,6 @@ fn socked_addr_roundtrip() {
             Token::U16(1234),
             Token::TupleEnd,
         ],
-        false,
+        Some(false),
     );
 }