configure `script/test` to not tolerate Ruby warnings

This makes CI fail if code is added to the project that generates
warnings in Ruby's verbose mode.
proper loading of minitest

blessed by @zenspider:
https://twitter.com/the_zenspider/status/348563962880733184
fix brittle URI parser tests failing on Rubinius

The tests relied on the output of Method#to_s, which seems unreliable on
non-MRI Rubies. Avoid that and ensure the default_uri_parser is restored.