[Testing] Test environment customization

[Cc618]

type: Feature

Description

It would be nice to have a setting to load a custom environment when running unit tests such as python.testing.env or support a test environment file such as .testenv (alongside a python.testing.envFile setting).
We can already override the environment using a .env file but the same file is used to run / debug non test code which might not be expected.

Example

Using Settings

"python.testing.env": {
  "VARIABLE": "42"
}

Using Custom File

.testenv file that will be loaded when running unit tests:

VARIABLE=42

[eleanorjboyd]

Hello! We are already working on this and considering it- here is the proposal, we would love your feedback on design if you have any thoughts: #21845.

closing as a duplicate of #17171 which is tagged in this issue