8355953: Add unit testing for internal jrtfs class prior to Valhalla work

[david-beaumont]

<draft PR for early feedback - do not review in depth yet unless asked>

This adds initial unit tests for the ExplodedImage class, but the aim is to make the helper able to create JImage images too, and then have tested run with both to show same behaviour.

I am going to be changing these classes soon(ish) as part of valhalla work, so having test coverage in mainline now is useful when refactoring later.

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Progress

• [ ] Change must be properly reviewed (1 review required, with at least 1 Reviewer)
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Issue

• JDK-8355953: Add unit testing for internal jrtfs class prior to Valhalla work (Enhancement - P4)

Reviewing

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Update a local copy of the PR:
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$ git pull https://git.openjdk.org/jdk.git pull/24962/head

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$ git pr checkout 24962

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https://git.openjdk.org/jdk/pull/24962.diff

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