Stop the created handler threads on the tests

The created handler thread will stay there for a while until
all the tests are finished. This is considered as a leakage
which will take the resource from the devices. In low end
devices, this may possibly impact the performance. The created
threads should be closed explicitly.

Test: atest ConnectivityCoverageTests ConntrackSocketTest
Change-Id: Ic1beb2a210e7c8c80c66fc9e0727c47599150672