[Logs Onboarding] Added api integration tests

[yngrdyn]

Relates to #159451.

This PR adds api integration tests to:

• GET /internal/observability_onboarding/custom_logs/privileges
• GET /internal/observability_onboarding/custom_logs/install_shipper_setup
• POST /internal/observability_onboarding/custom_logs/save

[apmmachine]

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[sorenlouv]

Lots of boilerplate but I suppose that's unavoidable when starting from scratch.
Tests look good, thanks!

[kibana-ci]

💚 Build Succeeded

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• Commit: 796dd22

Metrics [docs]

Unknown metric groups

ESLint disabled in files

id	before	after	diff
observabilityOnboarding	3	5	+2

ESLint disabled line counts

id	before	after	diff
enterpriseSearch	14	16	+2
securitySolution	416	420	+4
total			+6

Total ESLint disabled count

id	before	after	diff
enterpriseSearch	15	17	+2
observabilityOnboarding	12	14	+2
securitySolution	497	501	+4
total			+8

History

• 💔 Build #137988 failed e5a243669d52156cc4dfc549c33e39c7f82471d4
• 💚 Build #137007 succeeded 95009d8cd596cab94b100a22b684b4678a157429

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