RUM-6583: Fix flaky test for RUM CPU vital

[simaoseica-dd]

What and why?

The test testWhenCPUUnderHeavyLoadItMeasuresHigherCPUTicks() compares the CPU ticks under heavy and light processing and it started being flaky in GitLab. The CPU ticks measured should be greater on heavy processing.

This PR mitigates the flakiness of the test.

How?

• To avoid false positives, the blocks of processing are run more than once. This way we can have an average of measures that dilute the false positives.
• The duration of both tasks are now the same, to avoid comparisons of CPU ticks under different time intervals.
• It uses CFAbsoluteTimeGetCurrent instead of Date to get the time since it is more suitable for performance measurements providing high-precision timing.

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)
• Make sure each commit and the PR mention the Issue number or JIRA reference
• Add CHANGELOG entry for user facing changes
• Add Objective-C interface for public APIs (see our guidelines [internal]) and run make api-surface)

[mariedm]

Nice improvement! 🙌 Thank you for taking care of this flakiness. ❄

[datadog-datadog-prod-us1]

Datadog Report

Branch report: simaoseica/RUM-6583/fix-flaky-test-for-rum-cpu-vital
Commit report: 6f5d819
Test service: dd-sdk-ios

✅ 0 Failed, 3571 Passed, 0 Skipped, 2m 37.06s Total Time
🔻 Test Sessions change in coverage: 1 decreased (-0.02%), 7 increased, 6 no change

🔻 Code Coverage Decreases vs Default Branch (1)

• test DatadogCrashReportingTests tvOS 26.66% (-0.02%) - Details