test/rake: fix tests under test-all

* test/rake/test_rake_application.rb (test_display_exception_details):
  `RbConfig::CONFIG["prefix"]` can be the root directory, under
  test-all.

* test/rake/test_rake_test_task.rb (test_run_code_rake_default_gem):
  `-I` may be inserted, under test-all.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46890 b2dd03c8-39d4-4d8f-98ff-823fe69b080e