allow test_hec_event_writer to be executed more than once per splunk instance

[hsekowski-splunk]

If solnlib integration tests are called second time locally with use of the same splunk instance, error like below will be raised.
`

  assert len(search_results) == 1

E assert 2 == 1
E + where 2 = len([{'_bkt': 'main15806ABEBD-E78E-4B8A-B1B0-5DC0ADD73AB5', '_cd': '15:210', '_indextime': '1691056507', '_raw': '[{"tes...me": "test_hec_event_writes_with_non_utf_8", "field_a": "Üü_Öö_Ää_some_text", "field_b": "some_text_Üü_Öö_Ää"}]', ...}])
If [the SPL](https://github.com/splunk/addonfactory-solutions-library-python/blob/d7b36323d0070d6f04195d78e282cd7fa0494b28/tests/integration/test_hec_event_writer.py#L63) is extended by condition like | where _time>{test_start_timestamp} AND _time<{test_end_timestamp}`