Fixes #3767 (npm install overwrites npm link)

This adds a check for symlinks when installing
to prevent the install from overwriting any dependencies
that have been previously linked.

remove network access for tests
Fix outdated-json test on Windows.
Clean up ignore-install-link test.

It was using the incorrect npm instance, and had some weird system
where it passed the `t` throughout the test instead of letting errors
cause callback bailouts. Also, its use of raw child_process.spawn did
not work on Windows.