Skip RPM test if rpm2cpio is not present

Note: Arch's rpm2cpio (from rpmextract) doesn't work.
Fixed unit-test errors due to unpredictable set iteration order
Don't clear sys.meta_path in unit-tests

Python 3 seems to be using this for something, so just reset it back to what it
was before.
Fixed unit-test HTTP server's sending of redirects to work with Python 3.3