Use rspec -P pattern option in rake tasks

rspec has an internal option (-P) to receive pattern of files to run via the command line, use this option in rake_tasks instead of passing through a huge list of files one by one on the command line.
These weren't integration tests.  They weren't even particularly good unit tests. They were hitting a private API (using `__send__` to access `files_to_run`).

The consistent ordering is handled and tested elsewhere, and I don't see value in keeping these tests (or any like them).  If there are known holes in our test coverage and you want them covered by integration test, we can add some cucumber scenarios to cover them.  I don't see any holes here, though.