CliMulti: If process or output size is unexpected large, declare it as finished

#6617 Declares a process as finished as soon as a PID file is > 500bytes
(contains only PID) and output file is > 100MB (which should usually
not be larger than 100KB or 1MB) to prevent files growing up to many
GBs. I added unit and integration tests for the file size detection. It is
now possible to mock the methods file_exists and filesize although very
simple so far. Later we can allow to define callbacks to define different
return values for different files or we can use something like vfsStream