`use-test`: Allow import statements without default specifier

The `use-test` rule is triggered even if the import statement does not include a default specifier:

```ts
import type { TestInterface } from 'ava'; // "AVA should be imported as `test`."
```

Not a high priority issue I don't think, but it should be a quick fix, and ensure that `use-test` only concerns itself with `test`, not anything else.