add tests for syntax highlighting

tests/graphical.rs

@@ -1249,3 +1249,41 @@ fn primary_label() {
 
     assert_eq!(expected, out);
 }
+
+#[test]
+#[cfg(feature = "syntect-highlighter")]
+fn syntax_highlighter() {
+    #[derive(Debug, Error, Diagnostic)]
+    #[error("This is an error")]
+    #[diagnostic()]
+    pub struct Test {
+        #[source_code]
+        src: NamedSource,
+        #[label("this is a label")]
+        src_span: SourceSpan,
+    }
+
+    let err = Test {
+        src: NamedSource::new(
+            "hello_world.rs",
+            "fn main() {\n    println!(\"Hello, World!\");\n}\n".to_string(),
+        ),
+        src_span: (16, 26).into(),
+    };
+    let mut out = String::new();
+    GraphicalReportHandler::new_themed(GraphicalTheme::unicode())
+        .render_report(&mut out, &err)
+        .unwrap();
+    let expected = r#"\
+  × This is an error
+  ╭─[hello_world.rs:2:5]
+1 │ fn main() {
+2 │     println!(\"Hello, World!\");
+  ·     ─────────────┬────────────
+  ·                  ╰── this is a label
+3 │ }
+  ╰────
+"#;
+    println!("{}", out);
+    assert_eq!(out, expected)
+}