CI: Create job to run `native_unittests` on mandrel builds

[zakkak]

trafficstars

Follow up to https://github.com/oracle/graal/pull/6746

[zakkak]

Note that in Mandrel this is not as trivial as:

mx --primary-suite-path substratevm gate --strict-mode --tags native_unittests

it might require building/running the tests through mandrel-packaging

[github-actions[bot]]

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