* Add test-subj-selector module to packages dir.
* Rename to @kbn/test-subj-selector.
* Simplify rejectRejFiles Grunt task and ignore nested node_modules.
* Update jenkins_unit script to use yarn kbn CLI.
.rej are the byproduct of failed patches via git-apply or artifacts of
the backporting process through jasper. Any build that has them should
be rejected early.
