Add test

src/fill/level.rs

@@ -13,4 +13,17 @@ impl FillMode for LevelFillMode {
     fn sample(&self, level: f32, _: Vector) -> f32 {
         level
     }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn sample() {
+        let mode = LevelFillMode::new();
+
+        assert_eq!(0.3, mode.sample(0.3, Vector::ZERO));
+        assert_eq!(0.7, mode.sample(0.7, Vector::new(0.1, 0.2)));
+    }
 }