Limit the inclusion of some artifact comparisons in triage

[rylev]

As a follow up to #1281, this adds back some filtering to the triage report.

Specifically, artifact comparisons will only be included in the triage report if they fit one or more of the following conditions:

• 1 or more test result comparisons with medium or greater magnitude
• A combined number of test result comparisons for primary and secondary benchmarks that make the following true: (#primary * 2 + #secondary) >= 6.

This filters out a considerable number of artifact comparisons where only 1 or 2 small test result comparisons are relevant.

cc @Mark-Simulacrum @pnkfelix

[nnethercote]

r=me with the comment below addressed. Thanks.