Refactor OverlayTest to make it easier to test additional strategies.
Refactor OverlayTest to make it easier to test additional strategies.

Refactor to remove code duplication between OverlayTest and SandwichTest.
This is to make it easier to add tests for new strategies, such as the upcoming AllOrNothing strategy.

CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1372873004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#351392}

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