kernel-tests: modify stm dump tests

The default size of STM trace data is 64 bytes when no data is sent to
STM. It is sufficient to verify that trace data size is greater than
this size for valid data echoed to STM node. Modify STM dump tests
accordingly.

Change-Id: Iddd693976c28d66ee29abf62ef1a8493e3d71cdc
2 files changed