PeriodicAction unit test fails occassionally

The PeriodicAction unit test fails occasionally. Probably the sleep/check intervals between counter increments are not long enough.

See e.g. https://lhcb-nightlies.web.cern.ch/utils/test_report?url=https://gitlab.cern.ch/fwinkl/Gaudi/-/jobs/42655324/artifacts/raw/Test.xml:

-------------------------------------------------------------------------------
PeriodicAction
  manual start
-------------------------------------------------------------------------------
/builds/fwinkl/Gaudi/GaudiUtils/src/tests/PeriodicAction.cpp:40
...............................................................................

/builds/fwinkl/Gaudi/GaudiUtils/src/tests/PeriodicAction.cpp:50: FAILED:
  CHECK( counter == 2 )
with expansion:
  1 == 2