tests: kernel: work: fix uninitialized timer's work item

The `test_1cpu_drain_wait` tests iff the thread of a work queue
that is waiting to be drained can submit work item to the
queue. A timer is created with a callback funciton to submit a
work item to the same queue, to help demonstrate that behavior.

However, the work item submitted by `test_drain_wait_cb` to the
`coophi_queue` isn't initialized, and can hit assertion if it
is processed by the work queue.

Fixes #63559

Signed-off-by: Yong Cong Sin <ycsin@meta.com>