[Jruby] Make all tests in ActionView::TemplateDigestorTest green

This commit is so silly that's its cute!

22tests were failing as [`Dir.mktmpdir`](https://github.com/rails/rails/blob/master/actionview/test/template/digestor_test.rb#L26) doesn't seem to be working as expected. so this commit creates tmp dir & all tests in [TemplateDigestorTest](https://github.com/rails/rails/blob/master/actionview/test/template/digestor_test.rb) pass on JRuby :green_heart:

#11700