Use only uppercase DOCTYPE in tests where possible as required by the specification

See https://www.w3.org/TR/xml11/#NT-doctypedecl

Author: Mingun

tests/fuzzing.rs

@@ -51,7 +51,7 @@ fn fuzz_101() {
 
 #[test]
 fn fuzz_empty_doctype() {
-    let data: &[u8] = b"<!doctype  \n    >";
+    let data: &[u8] = b"<!DOCTYPE  \n    >";
     let mut reader = Reader::from_reader(data);
     let mut buf = Vec::new();
     assert!(matches!(

tests/reader-config.rs

@@ -471,7 +471,7 @@ mod trim_markup_names_in_closing_tags {
 }
 
 const XML: &str = " \t\r\n\
-<!doctype root \t\r\n> \t\r\n\
+<!DOCTYPE root \t\r\n> \t\r\n\
 <root \t\r\n> \t\r\n\
     <empty \t\r\n/> \t\r\n\
     text \t\r\n\