test/integration: synchronize scheduler shutdown to fix metrics data race

[mm4tt]

What type of PR is this?

/kind failing-test
/kind flake

What this PR does / why we need it:

This PR fixes a data race detected in TestSchedulerPerf (e.g., during preemption scenarios as reported in #137328).

The race occurred because legacyregistry.Reset() was being called at the end of a workload while the scheduler goroutine (launched via sched.Run(tCtx)) was still active. Even after workload evaluation is done, the scheduler could still perform a few more operations like updating metrics. If these updates happened concurrently with Reset(), it triggered the race detector.

Which issue(s) this PR is related to:

Fixes #137328.

Special notes for your reviewer:

Key changes:

• Refactored test/integration/util.StartScheduler into StartSchedulerWithDone. The new function returns a <-chan struct{} that is closed when the scheduler's goroutine actually terminates.
• Updated mustSetupCluster, setupTestCase, and setupClusterForWorkload in the scheduler_perf package to propagate this synchronization channel.
• Updated RunIntegrationPerfScheduling and RunBenchmarkPerfScheduling to explicitly wait for the scheduler to stop (<-schedulerDone) after cancelling the context and before calling legacyregistry.Reset().

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

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• d0f7cf3 test/integration: synchronize scheduler shutdown to fix metrics data race

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: mm4tt
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[mm4tt]

/assign @macsko

[macsko]

/test pull-kubernetes-scheduler-perf
/test pull-kubernetes-integration-race

[mm4tt]

/retest

[mm4tt]

/retest

[mm4tt]

/retest