test: fix test-repl-tab-complete.js

test-repl-tab-complete.js contains numerous assertions that are
never run. Anything that results in a ReferenceError bails out,
and never calls the functions containing the assertions. This
commit adds checking for successful tab completions, as well as
ReferenceErrors.

PR-URL: https://github.com/nodejs/io.js/pull/2052


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>