Edit environment variables programmatically in Unit Tests

**What is your question?**
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Environment variable and template tags are very powerful in Collections, but I don't find any document about their usage in Unit testing. Is it possible to use them in Unit testing?
In addition, I'd like to get some data from external apis to automate some testing scenarios, do Unit testing support it?

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To support table-driven testing, I'd like to define a static list of data before executing tests. In some special cases, these data can be got from external apis which I'd like to call to get that data to use, therefore, I need to call external api in unit testing.

The issue names no files, tests, or entry points. Start by locating the unit-testing functionality and its handling of environment variables and template tags, then determine whether external API calls are supported; done should be a documented answer or a clearly scoped implementation request.

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