Fix random failure spec

This is about the random failure specs for interruption with Ctrl-C
https://travis-ci.org/bbatsov/rubocop/jobs/6877252

The specs were testing real behavior by running rubocop
as a child process. However it seems to be unstable
and also cannot be measured in coverage report.
So rewrote them to run in thread with some stubs and mocks.
Merge pull request #121 from yujinakayama/fix-random-failure-spec

Fix random failure spec