Fix jar loading test in test/test_file.rb

This is a similar fix as for: https://github.com/jruby/jruby/commit/ed0b7ab1dfa76dee16ecfacbd07624a55b264c69

Generally, the workflow of adding foo/bar.jar to $LOAD_PATH and then expecting require 'bar' to load foo/bar.jar!/bar.rb doesn't work anymore. Instead, if you want it to search inside that jar, use foo/bar.jar!/