Below are some points based on my Elixir experience:

1. First of all I never heard about testing in dev environment. There was always a separate test environment that often has a completely different project configuration and test helpers that are available only for testing purposes.

2. I never heard about simulating remote location. First of all the code responsible for sending requests and read responses is in some package/library and it should have it’s own tests, so therefore you should not be responsible at all for testing it.

3. It’s rather not advised to rely on network connection. We have various of tools to avoid that. One example would be a request & response caching to the local drive, so tests needs to fetch specific page only once and after than every next call would be based on cached response. Alternatively you can use mocks and tell your code that you expect specific response, so you can focus on testing your app features and not the network.

Tests are often part of CI, so you are trying to slow down not only local development, but also verifying each commit in pull request. There are lots of possible network problems that you can’t predict and you really should not care about it at all.

Again usually you as a developer are testing not your network provider, but your application logic. It’s obvious that your application expects a working and stable connection to internet as otherwise it would not be able to serve pages. That’s why there is no reason and it’s why people are not testing it in every project.

However maybe you are in some very specific case. Without more context all we can do is to give a generic answers as we don’t know your use case. If it’s only about testing your API then usually nobody expects from you to write such tests and you usually should focus on testing your app features instead.