[onert/train] Add a mnist model validation test on CI

[zetwhite]

How about add a mnist model(#11389) to CI to make sure that onert_train is not broken?

• Good things to decide to do this work

  • How to validate training isn't broken

    • compare loss value between tf and onert-train
    • compare tensor dump between tf and onert-train
      • there is no way to dump onert-train tensor right now.

  • When will the tests be run?

    • every PR requested
    • daily

_{from #13209 (comment)}