[FormRecognizer] Tests: FormTrainingClient live tests not deleting trained models

[kinelski]

Description

Tests in FormTrainingClientLiveTests are not deleting the models trained for testing upon completion. The DisposableTrainedModel class is used for this purpose in FormRecognizerClientLiveTests.

Edit: some tests in OperationsLiveTests have the same issue.

Should we do the same in the FormTrainingClient live tests? Or would it be better to NOT wrap the StartTraining call in a separate class, making it an explicit call and relying on try/finally blocks to delete the models? The later would make sense in methods whose sole objective is testing the StartTraining method, while the former would be used in methods where training is only one of the steps of the test.