Get rid of "Then it should run without errors" step

Instead of this:

```
When I run `wp plugin status`
Then it should run without errors
And STDOUT should not be empty
```

It should just be:

```
When I run `wp plugin status`
And STDOUT should not be empty
```

As for command that are supposed to fail, we could have:

```
When I try `wp plugin status`
Then the return code should be 1
```