Fix incorrect test.

Fix incorrect test.

The packaging.install.remove function (a.k.a. the uninstall feature)
takes a path argument to allow client code to use custom directories
instead of sys.path.  The test used to give self.root_dir as path, which
corresponds to a prefix option, but prefix is not on sys.path, it’s only
the base directory used to compute the stdlib and site-packages
directory paths.  The test now gives a valid site-packages path to the
function.