Add more navbar items in tests/sites/base ?

[Carreau]

I think the tests are not exercising the dropdown in the navbar, as there are only 4 items in the tests build in tests/sites/base :

pydata-sphinx-theme/tests/sites/base/index.rst

Lines 8 to 11 in f30dec3

	page1
	page2
	section1/index
	google <https://google.com>

[Carreau]

Actually I did find a test that does test number of sidebar items (test_navbar_header_dropdown), but then I'm not sure why it does not fail in #1771 ...