Add a spec for saturate() in a plain CSS file

See sass/dart-sass#840

Author: nex3

spec/css/plain/functions.hrx

@@ -108,3 +108,16 @@ a {b: alpha(0.1)}
 a {
   b: alpha(0.1);
 }
+
+<===>
+================================================================================
+<===> saturate/input.scss
+@import "plain";
+
+<===> saturate/plain.css
+a {b: saturate(0.1)}
+
+<===> saturate/output.css
+a {
+  b: saturate(0.1);
+}