Ls with depth test fix

This is a newer version of ls-with-depth https://github.com/isaacs/npm/pull/4179
Updating the tests to avoid the cross-test corruption also found in https://github.com/isaacs/npm/pull/4212

The tests for these two --depth related cases both require fixtures that have mocked `node_modules` folders. However npm's prepublish script removes all `node_modules` folders in `/test` during previous tests, thus causing these two tests to fail.

I have renamed the fixtures to `node_modules_fixture`, and rename them back to `node_modules` during the tests only.

Full test suite passed on Mac OS X 10.9 with Node 0.10.22.