Implement/fix e2e bootstrap tests

[michel-laterman]

Our e2e tests have a couple of test suites which are currently disabled:

• https://github.com/elastic/fleet-server/blob/main/testing/e2e/agent_container_test.go
• https://github.com/elastic/fleet-server/blob/main/testing/e2e/agent_install_test.go

We should fix or reimplement these tests to ensure that the fleet-server repo has an automated test to verify that the bootstrap behaviour works as expected when running under the elastic-agent.

[cmacknz]

I moved this issue into the current sprint, assuming it is the simpler of elastic/elastic-agent#4769 to get working given the test already exists. Feel free to swap the order with elastic/elastic-agent#4769 if that isn't true.

[cmacknz]

Actually since elastic/elastic-agent#4643 isn't merged yet it would be nice to do elastic/elastic-agent#4769 as part of that work. Swapping now.