Add tests for a plain CSS @import url("foo") (#2033)

See sass/dart-sass#2397

spec/css/plain/import/in_css.hrx

@@ -1,19 +1,30 @@
-<===> input.scss
-@import "plain";
+<===> string/input.scss
+@use "plain";
 
-<===> plain.css
+<===> string/plain.css
 @import "whatever";
 
-<===> output.css
+<===> string/output.css
 @import "whatever";
 
-<===> warning
-DEPRECATION WARNING: Sass @import rules are deprecated and will be removed in Dart Sass 3.0.0.
+<===>
+================================================================================
+<===> url/unquoted/input.scss
+@use "plain";
 
-More info and automated migrator: https://sass-lang.com/d/import
+<===> url/unquoted/plain.css
+@import url(whatever);
 
-  ,
-1 | @import "plain";
-  |         ^^^^^^^
-  '
-    input.scss 1:9  root stylesheet
+<===> url/unquoted/output.css
+@import url(whatever);
+
+<===>
+================================================================================
+<===> url/quoted/input.scss
+@use "plain";
+
+<===> url/quoted/plain.css
+@import url("whatever");
+
+<===> url/quoted/output.css
+@import url("whatever");