test_integration_cmds: use a single teardown. (#475)

Instead of writing a custom ensure for every test let's just nuke
all the files every time. This may be something we might want to use for
other unit tests too. It leans heavily on the fact that a
`FileUtils.rm_rf` on files that don't exist is very quick and things
like `brew cleanup` are super slow in comparison.

Before:
```
$ brew tests --only=integration_cmds --official-cmd-taps
Finished in 49.764724s, 1.0047 runs/s, 5.2648 assertions/s.
```

After:
```
$ brew tests --only=integration_cmds --official-cmd-taps
Finished in 43.014769s, 1.1624 runs/s, 5.8352 assertions/s.
```