[ruby/fileutils] Fix test failure under ruby 2.4

`Exception#full_message` is only defined on ruby 2.5.0 and above.

https://github.com/ruby/fileutils/commit/a8968f41ed

tool/lib/test/unit/core_assertions.rb

@@ -381,7 +381,7 @@ eom
           msg = "exceptions on #{errs.length} threads:\n" +
             errs.map {|t, err|
             "#{t.inspect}:\n" +
-              err.full_message(highlight: false, order: :top)
+              RUBY_VERSION >= "2.5.0" ? err.full_message(highlight: false, order: :top) : err.message
           }.join("\n---\n")
           if message
             msg = "#{message}\n#{msg}"