Run K8s integration tests concurrently for each image variant

[rdner]

Describe the enhancement:
Currently, we run tests for each image sequentially

Describe a specific use case for the enhancement or feature:

If run tests for each image we produce publicly it takes about 1h 37 minutes, if we run these tests concurrently it might go down to ~40 minutes.

What is the definition of done?

All K8s integration tests are running concurrently and the runtime is down significantly.

It's a follow up to #5790

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[elasticmachine]

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