Fix backtrace features to not be false greens [#1469]

I took up fixing the issue from #1469.
Backtrace lines from files in `spec` are always printed with the backtrace exclusions, regardless of whether `--backtrace` is passed or not. (I believe this scenario would pass if `--backtrace` was not passed!)

The more interesting/useful thing about `--backtrace` is that _nothing_ is filtered.  This includes RSpec itself.  So, I think it might be worth updating this scenario description to:

````
Scenario: Running `rspec` with the `--backtrace` prints unfiltered backtraces
```

...and then adding a couple more steps on this scenario:

```
And the output should contain "lib/rspec/core"
And the output should contain "lib/rspec/expectations"
```

Thoughts?