Don't emit warning when the pattern of one-line pattern matching is just a variable pattern

https://github.com/ruby/dev-meeting-log/blob/master/DevelopersMeeting20201210Japan.md#feature-17371-reintroduce-expr-in-pat-ktsj

spec/ruby/language/pattern_matching_spec.rb

@@ -43,11 +43,11 @@ ruby_version_is "2.7" do
     describe "warning" do
       before do
         ruby_version_is ""..."3.0" do
-          @src = 'case 0; in a; end'
+          @src = 'case [0, 1]; in [a, b]; end'
         end
 
         ruby_version_is "3.0" do
-          @src = '1 => a'
+          @src = '[0, 1] => [a, b]'
         end
       end