Add unit tests for step timing function.

tests/unit/style/parsing/transition_timing_function.rs

@@ -25,11 +25,15 @@ fn test_cubic_bezier() {
 #[test]
 fn test_steps() {
     assert_roundtrip_with_context!(transition_timing_function::parse, "steps(1)");
+    assert_roundtrip_with_context!(transition_timing_function::parse, "steps(  1)", "steps(1)");
+    assert_roundtrip_with_context!(transition_timing_function::parse, "steps(1, start)");
+    assert_roundtrip_with_context!(transition_timing_function::parse, "steps(2, end) ", "steps(2)");
 
     // Step interval value must be an integer greater than 0
     assert!(parse(transition_timing_function::parse, "steps(0)").is_err());
     assert!(parse(transition_timing_function::parse, "steps(0.5)").is_err());
     assert!(parse(transition_timing_function::parse, "steps(-1)").is_err());
+    assert!(parse(transition_timing_function::parse, "steps(1, middle)").is_err());
 }
 
 #[test]

tests/unit/style/properties/serialization.rs

@@ -1210,6 +1210,19 @@ mod shorthand_serialization {
             assert_eq!(serialization, block_text);
         }
 
+        #[test]
+        fn transition_should_serialize_acceptable_step_timing_function() {
+            let block_text = "transition-property: margin-left; \
+                              transition-duration: 3s; \
+                              transition-delay: 4s; \
+                              transition-timing-function: steps(2, start);";
+            let block = parse(|c, i| Ok(parse_property_declaration_list(c, i)), block_text).unwrap();
+
+            let serialization = block.to_css_string();
+
+            assert_eq!(serialization, "transition: margin-left 3s steps(2, start) 4s;");
+        }
+
         #[test]
         fn transition_should_serialize_acceptable_frames_timing_function() {
             let block_text = "transition-property: margin-left; \