skip alpha tests by default for serial slow

[jsturtevant]

This will skip any alpha tests for serial slow jobs such as:

[Feature:SchedulerAsyncPreemption] [FeatureGate:SchedulerAsyncPreemption] [Alpha]

https://prow.k8s.io/view/gs/kubernetes-ci-logs/logs/ci-kubernetes-e2e-capz-master-windows-serial-slow/1891895226952323072

[jsturtevant]

/assign @marosset @zylxjtu

[k8s-ci-robot]

@jsturtevant: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-e2e-capz-windows-2025-extension	c4dd18e	link	false	/test pull-e2e-capz-windows-2025-extension

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[marosset]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jsturtevant, marosset

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