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.
Any tests that check for order when using ```t.pattern="..."``` belong in rspec core, not in the rake task. Since the ```FileList``` is no longer being used to parse the pattern in the rake task, these tests are not relevant anymore.

Note that when ```files_to_run``` is used (not ```pattern```), then there is still a spec that checks for consistent file ordering. So the file ordering is still there, and it is still being tested - just not for ```pattern```.