message: add testcase for `environ` shim
diff: @@ -8,4 +8,6 @@ fn main() {
  assert_eq!(env::var("MIRI_TEST"), Ok("the answer".to_owned()));
  // Test that miri environment is isolated when communication is disabled.
  assert!(env::var("MIRI_ENV_VAR_TEST").is_err());
  + // Test that the new variable is in the `std::env::Vars` iterator
  + assert!(env::vars().any(|(name, value)| name == "MIRI_TEST"))
  }
  