Various testing fixes

Convert STI test images to use busybox to minimize image size.

Change STI test image default script URLs to be served by localhost (and
start a local HTTP server in the integration tests) because otherwise
changes to STI that require corresponding changes to STI scripts can't
be tested before a git merge.

Update contrib/test to be runnable from anywhere.

Run STI integration tests from contrib/test.

Add closing of Reader used when downloading scripts.