ruby-ptest : some ptest fixes

ruby-ptest : some ptest fixes

- Skip the test_rm_r_no_permissions test under the root user, as
  deletion always succeeds.
- Filter out tests under the -ext- directory in run-ptest. Due to the
  commit [1],the packaging of .so test files under the .ext directory
  was removed. As a result, adjust the test filtering rules to avoid
  test failures caused by missing files.
- Add installation of rdoc.rb and did_you_mean.rb files in
  do_install_ptest to ensure complete test dependencies.
- Add init.rb file to PTEST installation path.

Signed-off-by: Jiaying Song <jiaying.song.cn@windriver.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>