Prevent a warning "possibly useless use of a literal in void context"

test/ruby/test_ast.rb

@@ -543,6 +543,9 @@ dummy
   end
 
   def test_encoding_with_keep_script_lines
+    # Stop a warning "possibly useless use of a literal in void context"
+    verbose_bak, $VERBOSE = $VERBOSE, nil
+
     enc = Encoding::EUC_JP
     code = "__ENCODING__".encode(enc)
 
@@ -553,6 +556,9 @@ dummy
 
     node = RubyVM::AbstractSyntaxTree.parse(code, keep_script_lines: true)
     assert_equal(enc, node.children[2].children[0])
+
+  ensure
+    $VERBOSE = verbose_bak
   end
 
   def test_e_option