[TECH DEBT] Returner API tests

This is going to expose and probably fix a ton of bugs. https://github.com/saltstack/salt/issues/48734 and https://github.com/saltstack/salt/issues/61729 at minimum.

Similar to how we added functional tests for the cache API we need similar testing for the returner API. There are quite clearly several returner issues that are probably all just simply due to the lack of a functional test suite.

So this issue is to track that effort.

Here are a list of the current returners:

• - appoptics_return.py - NOTE: this is a 3rd party API/SaaS and may require some test credentials in order to test
• - carbon_return.py
• - cassandra_cql_return.py
• - cassandra_return.py this module is obsolete and relies on a very old/no-longer-supported library
• - couchbase_return.py
• - couchdb_return.py
• - django_return.py this probably never worked. Marked for deprecation in #62644
• - elasticsearch_return.py
• - etcd_return.py
• - highstate_return.py
• - influxdb_return.py
• - kafka_return.py
• - librato_return.py
• - local_cache.py
• - local.py
• - mattermost_returner.py
• - memcache_return.py
• - mongo_future_return.py
• - mongo_return.py
• - multi_returner.py
• - mysql.py
• - nagios_nrdp_return.py
• - odbc.py
• - pgjsonb.py
• - postgres_local_cache.py
• - postgres.py
• - pushover_returner.py
• - rawfile_json.py
• - redis_return.py
• - sentry_return.py
• - slack_returner.py
• - slack_webhook_return.py
• - sms_return.py
• - smtp_return.py
• - splunk.py
• - sqlite3_return.py
• - syslog_return.py
• - telegram_return.py
• - xmpp_return.py
• - zabbix_return.py (see #62055 for a rewrite)

Start by reviewing the functional cache API testing work referenced in issue #61081 and the listed returner modules in Salt. Compare the existing checked returner coverage with the unchecked modules, including any constraints noted for third-party services. Done means the returner API has functional tests covering the intended current returners.