Fix starting and stopping of nksip_store in eunit test

This normalises the start and stopping of the process in the case that
we already have a running server before the test starts.  At this
stage, it seems to resolve the sporadic test failures in R16.
Merge pull request #13 from danielwhite/fix-nksip-store-test-cleanup

Fix starting and stopping of nksip_store in eunit test