SC2157 should also check `test`

```sh
#!/bin/sh

test -z SOMEARG  && echo y || echo n

[ -z SOMEARG ] && echo y || echo n
```

The `[ -z SOMEARG ]` line emits `SC2157 (error): Argument to -z is always false due to literal strings.`, as expected. However, the version with `test` does not get checked.