allow test_hec_event_writer to be executed more than once per splunk instance

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}`

Start with tests/integration/test_hec_event_writer.py, especially the SPL at line 63, and reproduce the test twice against the same Splunk instance. Check how test_start_timestamp and test_end_timestamp can distinguish events from the current run; done means a repeated execution returns only the current test event and keeps the assertion at one result.