[Fleet] Set up Fleet functional test suite

[jen-huang]

We need to do some bootstrapping work to set up functional tests for Fleet. This includes:

• Set up and configure x-pack/test/fleet_functional test directory
• Write a few basic tests to use as reference for future tests

Resources:
Kibana functional test docs
Existing functional test suites, such as Endpoint functional tests

[elasticmachine]

Pinging @elastic/fleet (Feature:Fleet)

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[paul-tavares]

Its great to see this issue created @jen-huang .
Once up and running, we should also consider moving some of the page_objects created under the Endpoint functional test setup over to Fleet (we would then import them from Fleet and use it in our tests). See here:

kibana/x-pack/test/security_solution_endpoint/page_objects/ingest_manager_create_package_policy_page.ts

Line 10 in 2ffdf75

export function IngestManagerCreatePackagePolicy({