Make the test suite compatible with --enable-frozen-string-literal

Ref: https://bugs.ruby-lang.org/issues/20205

Since `rexml` is tested as part of ruby-core CI, it needs to be compatible
with the `--enable-frozen-string-literal` option.

test/formatter/test_default.rb

@@ -2,7 +2,7 @@ module REXMLTests
   class DefaultFormatterTest < Test::Unit::TestCase
     def format(node)
       formatter = REXML::Formatters::Default.new
-      output = ""
+      output = +""
       formatter.write(node, output)
       output
     end