Fix pkg/units compilation of size_test

This fixes an issue introduced by #9233 

```
    /size_test.go:26: cannot use int64(float64(3.42 * GB)) (type int64) as type float64 in argument to HumanSize
    ./size_test.go:27: cannot use int64(float64(5.372 * TB)) (type int64) as type float64 in argument to HumanSize
    ./size_test.go:28: cannot use int64(float64(2.22 * PB)) (type int64) as type float64 in argument to HumanSize
```

The CI missed this issue.