[Fleet] Add Test coverage for preconfiguration variable override logic.

[kpollich]

Ref #113204
Ref elastic/fleet-server#742

Add automated test coverage for Fleet's preconfiguration and package_policy services that validates our variable overriding logic during setup.

The code path we need to hit is here, in our addPreconfiguredPolicyPackages method:

kibana/x-pack/plugins/fleet/server/services/preconfiguration.ts

Lines 370 to 379 in 7010b67

	await addPackageToAgentPolicy(
	soClient,
	esClient,
	installedPackage,
	agentPolicy,
	defaultOutput,
	name,
	description,
	(policy) => overridePackageInputs(policy, packageInfo, inputs)
	);

We should also add functional tests in the fleet_api_integration test project for the PUT /api/fleet/setup/preconfiguration API endpoint.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)