go_test: Add env attribute

[illicitonion]

What type of PR is this?

Feature

What does this PR do? Why is it needed?

This allows setting environment variables before static initialisers
run, and which reference expanded locations.

Which issues(s) does this PR fix?

Partially fixes #2921

[illicitonion]

I think this is ready for review :)

/cc @linzhp @robfig

[blico]

Thanks for working on this @illicitonion.

My understanding is that we could add support for this attribute using the TestEnvironment provider avoiding the need for a wrapper executable around go_test. https://docs.bazel.build/versions/main/skylark/lib/testing.html#TestEnvironment.

With this approach, we should just need to run whatever pre-processing is needed on the env attr (e.g. ctx.expand_location) and then construct a TestEnvironment provider and return it through go_test.

It would also be good to add some documentation to the go_test rule about the new attr, and some test cases verifying env is available during TestMain and the contents of env are expanded.

[illicitonion]

My understanding is that we could add support for this attribute using the TestEnvironment provider avoiding the need for a wrapper executable around go_test. https://docs.bazel.build/versions/main/skylark/lib/testing.html#TestEnvironment.

Oh nice, that's much simpler! Done!

It would also be good to add some documentation to the go_test rule about the new attr

Added to core.rst

and some test cases verifying env is available during TestMain and the contents of env are expanded.

There's one test (tests/core/go_test/env_test.go) showing that an env var is set and expanded to the path of a file which can then be read - are there more contexts you have in mind to test?