Allow running the standalone Beats serverless integration tests on a daily schedule

[cmacknz]

#3258 adds various tests for standalone Beats interacting with Serverless projects. These tests were added to the Elastic Agent repository because the Beats CI system is not currently capable of starting serverless projects for testing. While we expect this to be possibly eventually, in the meantime we should regularly run these tests to ensure we can catch regressions.

The scope of this issue is to add a Buildkite job that runs only the standalone Beats serverless integration tests as a nightly test suite. This should gate the merge of #3258 since these tests should not be triggered for each PR in this repository since the code here cannot cause regressions in these tests.

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[cmacknz]

We do not need this work once elastic/beats#36675 is completed, this issue only exists as a stop gap solution while we migrate the tests to Beats. We can avoid doing this at all if we can complete elastic/beats#36675 in the near future instead.