[crac] RFR: 8361090: [CRaC] Make dev engines less confusing

[Timofei Pushkin]

On Mon, 30 Jun 2025 17:21:50 GMT, Timofei Pushkin <tpushkin at openjdk.org> wrote:

> - Makes `pauseengine` Linux-only since it is not fully implemented on MacOS and Windows
>    - Also moves `criuengine` sources from `unix` to `linux` directory since it only supports Linux anyway
> - Makes `simengine` and `pausengine` report errors and status in more details

test/jdk/jdk/crac/recursiveCheckpoint/Test.java line 29:

> 27:  * @test
> 28:  * @summary check that the recursive checkpoint is not allowed
> 29:  * @requires (os.family == "linux")

I believe this test does not need `pauseengine` at all and there even may be errors because of it, but I am going to test this internally first and create a separate PR for that later

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PR Review Comment: https://git.openjdk.org/crac/pull/239#discussion_r2175560769