Qt: test_ml_model: test failure

From a60ff650, random failure:

PASS   : TestVLCMLModel::initTestCase()
PASS   : TestVLCMLModel::testModelRecuseUpdateFromSlot()
FAIL!  : TestVLCMLModel::testGetIndexFromID() 'QTest::qWaitFor([&row]() { return row != -1; }, timeout)' returned FALSE. ()
   Loc: [../../../../modules/gui/qt/tests/test_ml_model.cpp(193)]
PASS   : TestVLCMLModel::cleanupTestCase()
Totals: 3 passed, 1 failed, 0 skipped, 0 blacklisted, 2985ms
********* Finished testing of TestVLCMLModel *********
FAIL test_ml_model (exit status: 1)

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