core: fix buggy test TestContext2Input_providerMulti

This was trying to test gathering input from a default and an alias
provider configuration, but it was incorrectly setting the provider ref
on the instance that was supposed to belong to the alias. This was working
before because Terraform would gather input from the aliased provider
anyway, but now the invalid "alias" argument in the resource is producing
a validation error.

This doesn't actually make the test work again yet, because we still have
provider input disabled at this time, pending a forthcoming change to
how provider input is handled.

terraform/test-fixtures/input-provider-multi/main.tf

@@ -1,9 +1,9 @@
 provider "aws" {
-    alias = "east"
+  alias = "east"
 }
 
 resource "aws_instance" "foo" {
-    alias = "east"
+  provider = aws.east
 }
 
 resource "aws_instance" "bar" {}