Fixes a bug introduced in 0aa58b9 where we're writing a test file to
a taskdir-interpolated location, which works when we `alloc exec` but
not in the jobspec for a group script check.
This changeset also makes the test safe to run multiple times by
namespacing the file with the alloc ID, which has the added bonus of
exercising our alloc interpolation code for group script checks.
- In script checks, ensure we're running `Exec` against the new running
allocation and not the earlier stopped one.
- In script checks, allow `Exec` calls to error due to lack of pty when
we use the exec to kill the task.
- In `utils.go/RegisterAllocs`, force query for allocations to wait on
wait index returned by registration call.
