chore(notifications): Add SendTestNotification to __init__.py (#95174)

this might be why we are seeing logs where
`issue-send-test-notification-visible` isn't being registered. also
removes a duplicate from the list

Author: roggenkemper

src/sentry/notifications/notification_action/__init__.py

@@ -1,6 +1,5 @@
 __all__ = [
     "IssueAlertRegistryHandler",
-    "SlackIssueAlertHandler",
     "MetricAlertRegistryHandler",
     "AzureDevopsIssueAlertHandler",
     "DiscordIssueAlertHandler",
@@ -25,6 +24,7 @@
     "PluginActionHandler",
     "WebhookActionHandler",
     "SentryAppActionHandler",
+    "SendTestNotification",
 ]
 
 from .action_handler_registry import (
@@ -33,6 +33,7 @@
     WebhookActionHandler,
 )
 from .group_type_notification_registry import IssueAlertRegistryHandler, MetricAlertRegistryHandler
+from .grouptype import SendTestNotification
 from .issue_alert_registry import (
     AzureDevopsIssueAlertHandler,
     DiscordIssueAlertHandler,