Add support for Test / envVars

This may be out of scope for an issue, as there may be a workaround that I’m not aware of.

I want to set environment variables in tests:

Test / envVars := Map(
  "MY_TEST" -> "true",
)

Which are then accessible via:

global.process.env.selectDynamic("MY_TEST").asInstanceOf[js.UndefOr[String]]

For the JVM this works in tandem with Test / fork := true, however it seems that this is an option that is banned on top of Scala.js:

[error] `test / test` tasks in a Scala.js project require `test / fork := false`.

I’m guessing this happens because Scala.js has to fork a process anyway, but it appears that the forking ignores the envVars setting. Would be great if it didn’t.

[gzm0]

I have started on this, but likely will not have time any time soon. So feel free to take it over.

[sjrd]

@gzm0 Do you still want to do the sbt plugin side of this feature, now that the JSEnv side is done? Or would you prefer that I do it?

I still have a PR to actually (finally) separate the javalib in its own jar in progress, but otherwise I think we’ll be done for 1.11.0.