Problem: RPL.RPL_DUMP_THREAD_KILL is failig frequently in test runs.

Analysis: When this test script is running in combination with other tests,
it is possible that dump threads from those tests are not killed.
Every test script has rpl_end.inc but that .inc does not kill dump threads.
This test script expects that the number of dump threads are zero before
starting the test.

Fix: Doing cleanup (stopping dump threads) before the actual test starts.