Werk #13507: Fix simulation mode in service discovery and connection tests

Werk #13505 fixed the simulation mode for already existing hosts with existing cache.

If a host has no cache, like new added hosts, the "Check_MK Discovery" service will now show an informational output "Got no data (Simulation mode enabled and no cachefile present)", like in previous 1.6 versions. Furthermore, no service discovery will be processed.

While executing connection tests in simulation mode, the "Agent" section will use the cache file, if present, or is showing the above mentioned message. The SNMP datasources will not be contacted any more if simulation mode is enabled.

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