indent all extra failure lines correctly in system tests

If the output from Rails' system test teardown is multiple lines, we should try and render all of the lines with proper indentation.

This can happen on `rails/rails@master` now that failure screenshots can include the page HTML (rails/rails@36545), and can actually happen with v6.0.2.2 if the `method_name` ends up being a little too long.

lib/rspec/rails/example/system_example_group.rb

@@ -114,7 +114,8 @@ def driven_by(driver, **driver_options, &blk)
             original_after_teardown.bind(self).call
           ensure
             myio = $stdout
-            RSpec.current_example.metadata[:extra_failure_lines] = myio.string
+            myio.rewind
+            RSpec.current_example.metadata[:extra_failure_lines] = myio.readlines
             $stdout = orig_stdout
           end
         end