Support absolute path patterns.

While this wasn't officially supported previously,
setting `rake_task.pattern` to an absolute path pattern
in RSpec 3.0 and before worked since it delegated to
`FileList` internally (but now just forwards the pattern
on to the `rspec` command).

Fixes #1721.
Simplify `contain_files` matcher.

No need for `flatten`.  This also makes the call
sites less noisy by removing the array brackets.