Book tests disrespect CleanUrls disabled settings.

[Gormartsen]

See code:
https://github.com/backdrop/backdrop/blob/92026349212b0383a006f12a19ce5f0be0099e37/core/modules/book/tests/book.test#L160

https://github.com/backdrop/backdrop/blob/92026349212b0383a006f12a19ce5f0be0099e37/core/modules/book/tests/book.test#L165

https://github.com/backdrop/backdrop/blob/92026349212b0383a006f12a19ce5f0be0099e37/core/modules/book/tests/book.test#L170

    // Check previous, up, and next links.
    if ($previous) {
      $prev_link = '<a href="/node/' . $previous->nid . '" rel="prev" title="Go to previous page"><b>‹</b> ' . $previous->title . '</a>';
      $this->assertRaw($prev_link, 'Previous page link found.');
    }

    if ($up) {
      $up_link = '<a href="/node/' . $up->nid . '" title="Go to parent page">up</a>';
      $this->assertRaw($up_link, 'Up page link found.');
    }

    if ($next) {
      $next_link = '<a href="/node/' . $next->nid . '" rel="next" title="Go to next page">' . $next->title . ' <b>›</b></a>';
      $this->assertRaw($next_link, 'Next page link found.');
    }

[bradbulger]

This test also fails with clean URLs enabled when running in a subdirectory.

[indigoxela]

Maybe run it through url()?

Quick example: url('node/' . $next->nid)

That should take care of both, Backdrop in subdirectory and clean urls off.