Add css tests

src/main.rs

@@ -429,3 +429,41 @@ fn main() -> io::Result<()> {
     // drop(input);
     Ok(())
 }
+
+#[cfg(test)]
+mod test_css {
+    use super::StyleSheet;
+    macro_rules! test {
+        ($func:ident, $input:literal) => {
+            #[test]
+            fn $func() {
+                let mut buf = Vec::new();
+                StyleSheet::new($input)
+                    .print_as_css(&mut buf)
+                    .expect(concat!("failed to pretty print on ", $input));
+                assert_eq!(
+                    String::from($input),
+                    String::from_utf8(buf).expect("produced invalid utf8")
+                );
+            }
+        };
+        ($func:ident, $input:literal, $output:literal) => {
+            #[test]
+            fn $func() {
+                let mut buf = Vec::new();
+                StyleSheet::new($input)
+                    .print_as_css(&mut buf)
+                    .expect(concat!("failed to pretty print on ", $input));
+                assert_eq!(
+                    String::from($output),
+                    String::from_utf8(buf).expect("produced invalid utf8")
+                );
+            }
+        };
+    }
+
+    test!(nesting_el_mul_el, "a, b {\n  a, b {\n  color: red\n}\n}\n", "a a, b a, a b, b b {\n  color: red;\n}\n");
+    test!(basic_style, "a {\n  color: red;\n}\n");
+    test!(two_styles, "a {\n  color: red;\n  color: blue;\n}\n");
+    test!(selector_mul, "a, b {\n  color: red;\n}\n");
+}