This PR fix issue bareos/internal#189 warning with cmake 3.31 in
cmake_parse_arguments.

-- ✓ system:spool (baseport=30591)
--      └─ ✓ system:spool
CMake Warning (dev) at systemtests/cmake/BareosSystemtestFunctions.cmake:673 (cmake_parse_arguments):
  The COMMENT keyword was followed by an empty string or no value at all.
  Policy CMP0174 is not set, so cmake_parse_arguments() will unset the
  ARG_COMMENT variable rather than setting it to an empty string.
Call Stack (most recent call first):
  systemtests/cmake/BareosSystemtestFunctions.cmake:846 (add_disabled_systemtest)
  systemtests/tests/stresstest/CMakeLists.txt:21 (create_systemtest)
This warning is for project developers.  Use -Wno-dev to suppress it.

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