Change test blob count from 2 to 1

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Per investigation #343554 (comment 710869116), there should only be 1 blob of image now instead of 2. Updating test expectation to match with findings as well as changing to use image alpine@sha256:c3d45491770c51da4ef58318e3714da686bc7165338b7ab5ac758e75c7455efb instead of alpine:latest for a more stable outcome.

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Related to #343554 (closed)