gitignore: ignore test-npm artifacts

[Fishrock123]

See #1549, these files are produced while running make test-npm.

R=@othiym23

[othiym23]

This could be landed, but I think a better solution would be to fix the relevant test(s) in npm to not treat fixtures as mutable, and to update the test-npm stanza in the io.js Makefile to clean up the .bin directory after running the tests. .bin doesn't need to be included for distribution (which is part of why the release tarball doesn't include it). Rather than clutter the build tree, we should keep working on this until the test suite is idempotent.

[Fishrock123]

Ok, doing that. some make test-npm cleanup is at #1576, and I'll look into fixing that test from npm.