Fix formatting of \0 in tests by changing it to \u{1}

This follows this PR in rustc:
<rust-lang/rust#95345>

Which breakage was discussed here:
<rust-lang/rust#95732>

Still relying on \0 would require bumping the MSRV to 1.61.0

tests/integration/cdata.rs

@@ -68,8 +68,8 @@ test!(cdata_09, "<p><![CDATA[bracket ]after]]></p>",
     Token::ElementEnd(ElementEnd::Close("", "p"), 29..33)
 );
 
-test!(cdata_err_01, "<p><![CDATA[\0]]></p>",
+test!(cdata_err_01, "<p><![CDATA[\u{1}]]></p>",
     Token::ElementStart("", "p", 0..2),
     Token::ElementEnd(ElementEnd::Open, 2..3),
-    Token::Error("invalid CDATA at 1:4 cause a non-XML character '\\u{0}' found at 1:13".to_string())
+    Token::Error("invalid CDATA at 1:4 cause a non-XML character '\\u{1}' found at 1:13".to_string())
 );

tests/integration/elements.rs

@@ -211,7 +211,7 @@ test!(attribute_err_05, "<c a='<'/>",
 
 test!(attribute_err_06, "<c a='\0'/>",
     Token::ElementStart("", "c", 0..2),
-    Token::Error("invalid attribute at 1:3 cause a non-XML character '\\u{0}' found at 1:7".to_string())
+    Token::Error("invalid attribute at 1:3 cause a non-XML character '\\0' found at 1:7".to_string())
 );
 
 test!(attribute_err_07, "<c a='v'b='v'/>",

tests/integration/pi.rs

@@ -118,8 +118,8 @@ test!(declaration_err_12, "<?xml version='1.0'?><?xml version='1.0'?>",
     Token::Error("unknown token at 1:22".to_string())
 );
 
-test!(declaration_err_13, "<?target \u{0000}content>",
-    Token::Error("invalid processing instruction at 1:1 cause a non-XML character '\\u{0}' found at 1:10".to_string())
+test!(declaration_err_13, "<?target \u{1}content>",
+    Token::Error("invalid processing instruction at 1:1 cause a non-XML character '\\u{1}' found at 1:10".to_string())
 );
 
 test!(declaration_err_14, "<?xml version='1.0'encoding='UTF-8'?>",