Test installation of files in mkmf

To support gems that require extensions using full paths (Ruby
Bug #9106) RubyGems will need to install extensions into two places.
The extension install directory and the lib directory.

This commit sets up the tests for this change.
Also install extensions files into lib/

This is one workaround for Ruby Bug #9106 where msgpack uses full
require paths to require extensions.  Now the files exist in both
places.