WebSearch: nonexisting record API test case fix

* Amends test case `test_search_engine_python_api_for_nonexisting_record()`
  which was showing problems on some Jenkins builds due to warning being
  emitted on stderr which led to xunit infinite recursion.  Using
  still-nonexisting record ID, but within mediumint unsigned bounds (16M)
  does not emit any warning and hence avoids this problem.  (closes #1692)

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>