Tests are absurdly slow due to massive network requests

[wjt]

The full test suite takes many minutes to run, and performs multiple gigabytes of network requests. The worst offender is test_chromiumchecker.py because this test involves downloading a chromium source tarball – more than 3 GiB.

[wjt]

I tried to use aioresponses https://github.com/pnuckowski/aioresponses but I am apparently not smart enough to make this work. The mock object gets stopped too early for reasons I don't understand.