Detect historical flakes in passing TestGrid tabs

[SaaiAravindhRaja]

Fixes #22

Signalhound currently skips table data for tabs whose TestGrid summary has returned to passing. That hides failures still present in the tab history, which is common for hourly jobs after a short run of successful builds.

This change also inspects passing tabs and reports them as flaky only when their table history contains actual failure statuses. Table requests are bounded so the additional lookups do not run serially or without a limit.

Verification

• make test
• go test -race ./internal/testgrid ./cmd

[kubernetes-prow]

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