Proposed commit message

Add support in the CI pipeline for Pull Requests to run system tests with different Elastic Agent docker images.

This is achieved by setting the ELASTIC_AGENT_IMAGE_REF_OVERRIDE environment variable, as mentioned in the docs: https://github.com/elastic/elastic-package/blob/main/docs/howto/custom_images.md#use-your-own-custom-images

Added temporarily a log message to show the base Elastic Agent docker image used in tests:

Base Elastic Agent image: docker.elastic.co/elastic-agent/elastic-agent:8.13.0
Base Elastic Agent image: docker.elastic.co/elastic-agent/elastic-agent:8.16.0-SNAPSHOT

Currently, by default elastic-package uses: docker.elastic.co/elastic-agent/elastic-agent-complete

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• [ ] I have verified that all data streams collect metrics or logs.
• [ ] I have added an entry to my package's changelog.yml file.
• [ ] I have verified that Kibana version constraints are current according to guidelines.

Author's Checklist

• Test Elastic Agent docker image override in stack and tests.

How to test this PR locally

The following steps will be performing the same changes as in this PR:

export ELASTIC_AGENT_IMAGE_REF_OVERRIDE=docker.elastic.co/elastic-agent/elastic-agent:8.16.0-SNAPSHOT

# This command should run Elastic Agent and Fleet Server with the docker image
# defined above instead of using the complete docker image
elastic-stack up -v -d --version 8.16.0-SNAPSHOT

# Check docker images
docker ps --format "{{.Names}} {{.Image}}" |grep "elastic-agent"

cd packages/nginx

# Run system tests to trigger new Elastic Agents
elastic-package test system -v 

# While running the system tests , check the docker images used by the Elastic Agent containers
docker ps --format "{{.Names}} {{.Image}}" |grep "elastic-agent"

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