Add a basic prefill test in maxengine

[liurupeng]

Description

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[patemotter]

Your test is currently failing FAILED tests/maxengine_test.py::MaxEngineTest::test_basic_prefill - AssertionError: Array([[15028]], dtype=int32) != Array([31398], dtype=int32).

To fix the linter issue you should run bash code_style.sh within MaxText and it should format your file correctly.