Refactor harness

[zkoopmans]

It is a bit much to have a harness struct and methods in the harness. Move to either/or not both.

• Methods like WaitUntilServing and DropCaches should be dealt with.
• Rename harness declarations in all test packages.
• Consider making the harness a const in declarations and get rid of Main methods.

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This issue is stale because it has been open 90 days with no activity. Remove the stale label or comment or this will be closed in 30 days.

[github-actions]

There are TODOs still referencing this issue:

1. test/benchmarks/harness/util.go:30: move to own package or move methods to harness struct.

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A friendly reminder that this issue had no activity for 120 days.

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This issue has been closed due to lack of activity.