gui: Fix return code assert in CpuConsumer_test.

The CpuConsumer error code was changed recently from INVALID_OPERATION to
NOT_ENOUGH_DATA, however the CpuConsumer_test was not updated. This change
fixes the return code checked by CpuConsumer_test so the test no longer
fails.

Change-Id: Ie548ddcb313fa34c130ac63355090cb44ac15ebf