services/device/internal/impl/utiltest/helpers.go: fewer leaked processes
services/device/internal/impl/utiltest/helpers.go: fewer leaked processes

Make some minor changes to the device manager tests to improve
the likelyhood of successfully cleaning up processes if the tests fail.

Change-Id: I12e424845a0bbca4a70cb1d076dfee43b669c5eb

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref