Make breadcrumb test more robust

[Pander]

In this test https://github.com/greyli/bootstrap-flask/blob/master/tests/test_render_breadcrumb_item.py#L14 the HTML list item element contains two consecutive spaces between its attributes.

I would expect the created HTML to contain only one space, see https://getbootstrap.com/docs/4.1/components/breadcrumb/ Hence, this looks like a test that might fail easily if the number of spaces change.

Should the test be more robust and accept one and two spaces between the attributes?