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>