fix concurrent test relying on sleep
fix concurrent test relying on sleep

Make an old concurrent test deterministic, and not rely on sleep for any
synchronization.

hashicorp/terraform,ewbankkit/terraform,apparentlymart/terraform,ewbankkit/terraform,tommynsong/terraform,apparentlymart/terraform,tommynsong/terraform,rnaveiras/terraform,ewbankkit/terraform,ewbankkit/terraform,tommynsong/terraform,rnaveiras/terraform,hashicorp/terraform,apparentlymart/terraform,rnaveiras/terraform,hashicorp/terraform,apparentlymart/terraform,apparentlymart/terraform