behat: allow inner calls to `wp`

With the current setup, the following step would fail:

```
When I run `echo 'SHOW TABLES;' | wp db`
```

That's because the Behat steps expect commands to start with `wp `.

One option would be to add some fancy regex to replace `wp` with the absolute path.

~~Or we could define a temporary alias and let bash do its thing.~~ Bash aliases are available only in interactive mode.

We can use a bash function.