dbjobsource: fix test for multiple health checks
dbjobsource: fix test for multiple health checks

Device#initiate_health_check_job() will never return None. Instead we
check device.get_existing_health_check_job first, then only proceed to
create a health check if there is none already running or waiting.

Change-Id: Iffbf5b23a741a1ff3a0dfd808b8eee2fc88ab8c1

OSSystems/lava-server,Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,OSSystems/lava-server,OSSystems/lava-server,Linaro/lava-server