Don't exclude test directories

[iarna]

Adding new exclusions to what goes in published artifacts is breaking for npm and the code this replaces didn't exclude tests by default.

The breakingness is definitely a judgement call. I wouldn't view adding more exclusions for gyp temp files or editor cruft as breaking, for example.

[isaacs]

Weird, I seem to recall having some kind of reason for that, but it strikes me now as obviously wrong. Please do land this, lgtm, thank you.

[noahsilas]

Thanks for this fix - due to an unusual design decision some time ago, some private repos I'm working in are sharing some tests by putting them into a shared package's test directory, and trying to read those tests in the dependant repos is failing under npm 5.4.1.

In my particular case, I was able to add a .npmignore whitelist entry to the shared repo (!test), but that meant that I had to cut new versions of that repo and update that dependency at the same time as moving up to npm 5.4. 😅