Fix plain CSS tests

Author: nex3

spec/css/plain/function.hrx

@@ -1,4 +1,7 @@
-<===> lowercase/parameter/input.css
+<===> lowercase/parameter/input.scss
+@use "plain";
+
+<===> lowercase/parameter/plain.css
 @function --a(--b <color>) {result: c}
 
 <===> lowercase/parameter/output.css
@@ -8,7 +11,10 @@
 
 <===>
 ================================================================================
-<===> lowercase/returns/input.css
+<===> lowercase/returns/input.scss
+@use "plain";
+
+<===> lowercase/returns/plain.css
 @function --a() returns <ident> {result: b}
 
 <===> lowercase/returns/output.css
@@ -18,7 +24,10 @@
 
 <===>
 ================================================================================
-<===> lowercase/result/sass_script/input.css
+<===> lowercase/result/sass_script/input.scss
+@use "plain";
+
+<===> lowercase/result/sass_script/plain.css
 @function --a() {
   result: $b;
 }
@@ -30,7 +39,10 @@
 
 <===>
 ================================================================================
-<===> lowercase/result/characters/input.css
+<===> lowercase/result/characters/input.scss
+@use "sass";
+
+<===> lowercase/result/characters/plain.css
 @function --a() {
   result: {}#&%^*;
 }
@@ -42,7 +54,10 @@
 
 <===>
 ================================================================================
-<===> uppercase/result/sass_script/input.css
+<===> uppercase/result/sass_script/input.scss
+@use "sass";
+
+<===> uppercase/result/sass_script/plain.css
 @FUNCTION --a() {
   result: $b;
 }
@@ -54,7 +69,10 @@
 
 <===>
 ================================================================================
-<===> uppercase/result/characters/input.css
+<===> uppercase/result/characters/input.scss
+@use "sass";
+
+<===> uppercase/result/characters/plain.css
 @FUNCTION --a() {
   result: {}#&%^*;
 }