Port the base service provider tests from Test::Unit.

The base service provider is terribly anæmic, but we had some tests, and it
does serve as the parent for almost every other provider. So, port those
tests over.

Also, add a dash of cross-platform support by invoking the current Ruby
interpreter to do file system manipulation things - rather than leaning on
Unix tools that are not available everywhere.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>