Add env attribute to go_test rule

[blico]

This change adds the env attribute to the go_test rule.

env is a map of environment variables that will be set before the test is run.

[jayconrod]

Why set environment variables in Bazel rules and not in the tests themselves?

If there's a reason environment variables must be set in rules, does it make sense for it to be a common attribute for all test rules in Bazel itself?

rules_go seems like the wrong place for this.

[Globegitter]

We have also run into cases where it is useful to set env vars before a target runs (not so much in go but especially js). I opened bazelbuild/bazel#7364 for that, @blico might be useful to +1 that issue as well :)

[Globegitter]

Btw currently as a workaround for our cases we are using the command rule from here: https://github.com/atlassian/bazel-tools/blob/master/multirun/README.md

[jayconrod]

This PR won't actually set environment variables before the target runs. Instead, they'll be set in main. init functions would not see the new environment variables. So I'd encourage people to write their own TestMain functions to set variable instead so it's clear what's happening. This would let the same tests work with go test, too.

bazelbuild/bazel#7364 would be able to set variables before the test executes. I think that's a better solution.

[blico]

Thank you for the feedback.

I will close this PR and look into bazelbuild/bazel#7364