test/ruby/test_pattern_matching.rb: Avoid a warning

http://rubyci.s3.amazonaws.com/ubuntu2004/ruby-master/log/20210119T033003Z.log.html.gz
```
/home/chkbuild/chkbuild/tmp/build/20210119T033003Z/ruby/test/ruby/test_pattern_matching.rb:798:
warning: assigned but unused variable - x
```

test/ruby/test_pattern_matching.rb

@@ -796,6 +796,7 @@ END
     assert_block do
       case [0, 1, 2]
       in x
+        x = x # avoid a warning "assigned but unused variable - x"
         true
       in [*, 2, *]
         false