Add test cases for transition-duration.

tests/unit/style/parsing/mod.rs

@@ -105,6 +105,7 @@ mod selectors;
 mod supports;
 mod text;
 mod text_overflow;
+mod transition_duration;
 mod transition_property;
 mod transition_timing_function;
 mod ui;

tests/unit/style/parsing/transition_duration.rs

@@ -0,0 +1,17 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use parsing::parse;
+use style::properties::longhands::transition_duration;
+
+#[test]
+fn test_positive_transition_duration() {
+    assert!(parse(transition_duration::parse, "5s").is_ok());
+    assert!(parse(transition_duration::parse, "0s").is_ok());
+}
+
+#[test]
+fn test_negative_transition_duration() {
+    assert!(parse(transition_duration::parse, "-5s").is_err());
+}