Remove duplicates to Vim 8.1 default runtime

This commit also changes file names and test code for maintainability.

after/syntax/css/masking-1.vim

@@ -0,0 +1,7 @@
+syn match cssFontProp contained "\<clip-\(path\|rule\)\>"
+syn match cssFontProp contained "\<mask\(-\(image\|mode\|repeat\|position\|clip\|origin\|size\|composite\|type\)\)\=\>"
+syn match cssFontProp contained "\<mask-border\(-\(source\|mode\|slice\|width\|outset\|repeat\)\)\=\>"
+syn keyword cssFontAttr contained nonzero evenodd alpha luminance add subtract intersect exclude
+syn match cssFontAttr contained "\<\(fill\|stroke\|view\)-box\>"
+syn match cssFontAttr contained "\<no-clip\>"
+syn match cssTagName "\<mask\>"