use method provided by minitest

The process of converting `Test` to `String` is already defined in minitest.
I think it is better to use that for the consistency of output content.

ref: https://github.com/seattlerb/minitest/blob/master/lib/minitest/test.rb#L261..L267

railties/lib/rails/test_unit/reporter.rb

@@ -18,7 +18,7 @@ def record(result)
       if output_inline? && result.failure && (!result.skipped? || options[:verbose])
         io.puts
         io.puts
-        io.puts format_failures(result).map { |line| color_output(line, by: result) }
+        io.puts color_output(result, by: result)
         io.puts
         io.puts format_rerun_snippet(result)
         io.puts
@@ -66,12 +66,6 @@ def format_line(result)
         "%s#%s = %.2f s = %s" % [result.class, result.name, result.time, result.result_code]
       end
 
-      def format_failures(result)
-        result.failures.map do |failure|
-          "#{failure.result_label}:\n#{result.location}:\n#{failure.message}\n"
-        end
-      end
-
       def format_rerun_snippet(result)
         location, line = result.method(result.name).source_location
         "#{self.executable} #{relative_path_for(location)}:#{line}"