fix: add Splunk HEC token validation for external Splunk (#877)

This PR adds validation of HEC token. It just checks if an event can be
send via given HEC token to external Splunk.
Fix was tested locally. The result for 3 workers are presented in the
attached screenshot (with pytest-splunk-addon code from develop branch
it breaks main worker , restarts it and wait indefinitely).
Although splunk_external fixture is defined in the session scope it runs
on each worker separately.
The following question arises: is it expected behaviour, or it's
something we'd like to change (for the fixture logic to be executed
before pytest splits for several processes). I do not believe that we
have to check if Splunk is responsive from each worker separately. That
would need to be explored (such solution might not be compatible with
pytest):

![image](https://github.com/user-attachments/assets/95bd1c7c-d6ed-43d7-8249-7475e4760808)

Author: kdoroszko-splunk

pytest_splunk_addon/splunk.py

@@ -639,6 +639,7 @@ def splunk_external(request):
             "Could not connect to Splunk HEC"
             "Please check the log file for possible errors."
         )
+    is_valid_hec(request, splunk_info)
     return splunk_info
 
 
@@ -1016,6 +1017,36 @@ def is_responsive(url):
         return False
 
 
+def is_valid_hec(request, splunk):
+    """
+    Verify if provided hec token is valid by sending simple post request.
+
+    Args:
+        splunk (dict): details of the Splunk instance
+
+    Returns:
+        None
+    """
+
+    LOGGER.info(
+        "Validating HEC token...  splunk=%s",
+        json.dumps(splunk),
+    )
+    response = requests.post(
+        url=f'{request.config.getoption("splunk_hec_scheme")}://{splunk["forwarder_host"]}:{splunk["port_hec"]}/services/collector/raw',
+        headers={
+            "Authorization": f'Splunk {request.config.getoption("splunk_hec_token")}'
+        },
+        data={"event": "test_hec", "sourcetype": "hec_token_test"},
+        verify=False,
+    )
+    LOGGER.debug("Status code: {}".format(response.status_code))
+    if response.status_code == 200:
+        LOGGER.info("Splunk HEC is valid.")
+    else:
+        pytest.exit("Exiting pytest due to invalid HEC token value.")
+
+
 def pytest_unconfigure(config):
     if PYTEST_XDIST_TESTRUNUID:
         if os.path.exists(PYTEST_XDIST_TESTRUNUID + "_wait"):