add tests to confirm the behavior when `format` and `extensions` are conflicted

If we have the file structure below

```
/hello
/hello.html
/hello/index.html
```

with options

```js
{
index: 'index.html',
extensions: ['.html']
}
```

No files or test paths are named. Start by locating the existing tests and the handling of `index` and `extensions`, then clarify the expected result for `/hello`, `/hello.html`, and `/hello/index.html` with the shown options. Done means tests explicitly cover the conflicting `format` and `extensions` behavior.

Written by the indexing model from the issue text.