While working on #20462, I discovered that some of the scheduler tests for
disconnected clients making long blocking queries. The tests used
`testutil.WaitForResult` to wait for an evaluation to be written to the state
store. The evaluation was never written, but the tests were not correctly
returning an error for an empty query. This resulted in the tests blocking for
5s and then continuing anyways.
In practice, the evaluation is never written to the state store as part of the
test harness `Process` method, so this test assertion was meaningless. Remove
the broken assertion from the two top-level tests that used it, and upgrade
these tests to use `shoenig/test` in the process. This will save us ~50s per
test run.
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