[Security Solution][Detections] Refactor functional update_actions tests for rules update API

[vitaliidm]

Background: #138625 (comment)

Summary

Current add actions tests for immutable rules have complex logic, that includes:

• fetching immutable rule
• update immutable rule with mocked properties
• add actions to it
• verify added action and rest of updated rule properties, if they were not modified, against earlier mocked rule

source: update_actions.ts

        const immutableRule = await getRule(supertest, log, '9a1a2dae-0b5f-4c3d-8305-a268d404c306');
        const hookAction = await createNewAction(supertest, log);
        const newRuleToUpdate = getSimpleRule(immutableRule.rule_id);
        const ruleToUpdate = getRuleWithWebHookAction(hookAction.id, false, newRuleToUpdate);
        const updatedRule = await updateRule(supertest, log, ruleToUpdate);
        const bodyToCompare = removeServerGeneratedProperties(updatedRule);

Proposal

• simplify tests by removing step of updating immutable rule with mock data
• make assertions whether rule properties were not modified against fetched earlier immutable rule

That would allow:

• to ease maintenance, as we won't need to keep mocks up to date with rule schema
• make tests more robust, as adding new fields to rule, won't cause tests failures (example: adding required_fields to rule causes tests failure, as mock expected empty results for it)

[elasticmachine]

Pinging @elastic/security-detections-response (Team:Detections and Resp)

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)

[vitaliidm]

implemented in #142464