show relative path the rerun snippet of test runner in rails engine

Since the absolute path is not required to re-run the test,
modified so that unnecessary information is not displayed.

```ruby
# before
bin/rails test /path/to/blorgh/test/integration/navigation_test.rb:5

# after
bin/rails test test/integration/navigation_test.rb:5
```

1 files changed, 5 insertions, 1 deletions

diff --git a/railties/lib/rails/test_unit/reporter.rb b/railties/lib/rails/test_unit/reporter.rb
index de4b002e55..00ea32d1b8 100644
--- a/railties/lib/rails/test_unit/reporter.rb
+++ b/railties/lib/rails/test_unit/reporter.rb
@@ -44,7 +44,7 @@ module Rails
     end
 
     def relative_path_for(file)
-      file.sub(/^#{Rails.root}\/?/, '')
+      file.sub(/^#{app_root}\/?/, '')
     end
 
     private
@@ -66,5 +66,9 @@ module Rails
 
         "#{self.executable} #{relative_path_for(assertion_path)}"
       end
+
+      def app_root
+        @app_root ||= defined?(ENGINE_ROOT) ? ENGINE_ROOT : Rails.root
+      end
   end
 end