8278851: Correct signer logic for jars signed with multiple digestalgs

[coffeys]

The manifest entry verifier exits too quickly in the case where multiple digests are used in a JarEntry and weak digests are present.

I've introduced some test test library functionality to help with jar signing. I've retrofitted one of my earlier tests with that new functionality also in this PR.

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Progress

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Issue

• JDK-8278851: Correct signer logic for jars signed with multiple digestalgs

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