AC-943: Create unit test for Magento2\Less\ColourDefinitionSniff check

Author: svera

Magento2/Tests/Less/ColourDefinitionUnitTest.less

@@ -6,6 +6,7 @@
 @red: #aaaaaa;
 @blue: #00F;
 @green: #0f0;
+@purple: #abcdef;
 
 .my{
   color: @red;

Magento2/Tests/Less/ColourDefinitionUnitTest.php

@@ -15,7 +15,7 @@ public function getErrorList()
         return [
             6 => 1,
             7 => 1,
-            15 => 1,
+            16 => 1,
         ];
     }