IMO having a test environment is a bit of a failure: you should be able to spin up and stop a full test environment easily, ideally (and feasibly for most systems) on the developer's local machine.
You need a testing environment, especially if there are other physical elements that are being integrated (embedded and other equipment).
Yeah, obviously this isn't gonna work for embedded development, but not that many people have that specific problem. Is it really so hard to spin up ephemeral testing clusters on a single machine? Everywhere that I've seen it done, it's a horrible kludge of scripts and hacks that's almost as difficult to understand as the code under test.
I write a lot of Terraform to manage internal infrastructure with dependencies on other internal and external infrastructure that's outside of my control. I define a module parameter for each dependency (usually with a default value that points to the real service URL) and then create a second module that uses Docker to spin up and configure local instances of each dependency and overrides the corresponding parameter.
This makes local testing very straightforward: `terraform -chdir=tests apply`
I work in Automatic Fare Collection (think railway passenger gates, ticket vending, contactless readers, bus driver consoles and readers etc as well as the backends that deal with the fare calculation and payments).
There are usually multiple vendors involved, and even with the best API and interface documentation and individual system testing against the specifications, you still need to bring the entire environment, including the hardware, together.
These ITFs (Integration Test Facilities) tend to be in existence for the life of the systems, including after hardware refreshes etc.
Yes you can test performance with jmeter etc, but sometimes you need 20 people walking in a circle between an entrance gate and an exit gate.
In past years I wouldn't say that, but these days it's so easy to get an LLM to write something like this plus a quick and dirty visualization.
I'm not saying this can be a replacement for a physical test setup, but it's so much easier to make changes when the tests can just give a clear cut "yes, this acts like you expect" or "no, this is broken."