test brotli nginx extension

[Theodlz]

Currently, we test that brotli is enabled correctly, in skyportal. But this is a baselayer feature, and it should be tested here instead. Similarly, it's a python dependency of skyportal, not of baselayer. I think it should be added here instead.

For reference, here's the skyportal unit test which checks that the server is able to brotli compress something: https://github.com/skyportal/skyportal/blob/main/skyportal/tests/frontend/test_brotli.py

PS: adding nginx with brotli support takes time in the SkyPortal CI that's already long enough. Moving tests here (where they belong anyways, since its a baselayer feature) would also help with that...