Add functional tests for insecure cluster warning

[jportner]

Starting in 7.16, we got rid of the securityOss plugin, refactoring the insecure cluster warning code in the process.

Before 7.16, the insecure cluster warning could be shown in two different scenarios:

1. The Kibana security plugin is not available
2. Kibana security is available, but Elasticsearch security is disabled

After 7.16, scenario 1 is no longer possible, because the code has been moved to the security plugin. If the security plugin is not available, the insecure cluster warning will not be shown at all.

As described in #113946 (comment), we never had functional test coverage for scenario 2, because our functional testing tooling did not support running the Kibana server in that fashion.

Once we have support for that in 8.0, we should add new tests to exercise scenario 2. They will be nearly identical to the tests that are mentioned in the linked comment above.

Blocked by: #54023 <-- this will include the work to change our security functional tests to run with the Kibana security plugin available and Elasticsearch security disabled

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)

[legrego]

Potentially unnecessary pending the outcome of #125364

[legrego]

@kc13greiner if you're feeling ambitious, I think it'd be great to address this issue alongside #116971. You'll probably need some help understanding the functional test runner, but either @azasypkin or I can assist when the time comes.