Run 7.9.x tests for ingest-manager daily

[mdelapenya]

We already have a regular pipeline per test suite (Helm, Integrations and Fleet), maybe we should convert them into a MBP, discarding PRs, and keeping existing timer-trigger.

[mdelapenya]

The use cases could be:

Run the e2e tests nightly for each maintenance branch

We currently do that for master, but new maintenance branches will eventually appear (7.9.x). We could:

• convert the regular pipelines for the scheduled jobs for the three test suites (Helm, integrations and Fleet) into MBP, filtering by head-filter-regex (See elastic/apm-integration-testing@8e66f2c)
• control the target job to be triggered (See https://github.com/elastic/beats-tester/blob/f2662a89f55296fd0249306d9970b991c18dab42/.ci/Jenkinsfile#L32)

Run the e2e tests right after the packaging job for a PR has finished

This way we could consume the artifacts generated by the packaging job (https://beats-ci.elastic.co/job/Beats/job/packaging/): binaries and docker images.

Thanks to @v1v for pointing out to real, existing examples.