(maint) Use real types and providers to exercise post_resource_eval

Previously, the spec tests were using the user type to test
post_resource_eval, but this has the side-effect of enumerating all of
the users on the system. On Mac OSX, this takes upwards of 4 seconds due
to parsing plist info.

This commit adds two new test-specific providers, which override the
post_resource_eval method, like a real type would do.