Register raftDropHeartbeat failpoint

[nwnt]

As a follow up of #20457, this registers the raftDropHeartbeat failpoint.

cc @serathius

[codecov]

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[serathius]

Awesome work! Thank you for finishing the failpoint improvements.

Unfortunately we started getting a lot of flakes.
/retest

[k8s-ci-robot]

@nwnt: The following tests 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-etcd-grpcproxy-integration-amd64	f345957	link	true	/test pull-etcd-grpcproxy-integration-amd64
pull-etcd-integration-4-cpu-amd64	f345957	link	true	/test pull-etcd-integration-4-cpu-amd64
pull-etcd-coverage-report	f345957	link	true	/test pull-etcd-coverage-report

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nwnt, serathius

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[serathius]

QQ, have you confirmed that RaftDropHeartbeat failpoint is triggered? For gofailSleepAndDeactivate we don't have a way to confirm that failpoint was really triggered. The issue #14729 was dropped.