Refactor tests that require direct access to model system index

[jmazanec15]

Description

Some of our rest tests make direct calls to the MODEL_SYSTEM_INDEX to initialize/check the state for a given test. However, the system index should not be directly accessed by the user for any reason. So, it does not make sense to access it for rest test cases. This can lead to flaky test behavior.

I did a quick scan of the tests that access the system index:
KNNRestTestCase.createModelSystemIndex:

1. RestDeleteModelHandlerIT.testDeleteModelExists
2. RestDeleteModelHandlerIT.testDeleteTrainingModel
3. RestDeleteModelHandlerIT.testDeleteModelFailsInvalid
4. RestDeleteModelHandlerIT.testTrainingDeletedModel
5. RestGetModelHandlerIT.testGetModelExists
6. RestGetModelHandlerIT.testGetModelExistsWithFilter
7. RestGetModelHandlerIT.testGetModelFailsInvalid
8. RestGetModelHandlerIT.testGetModelFailsBlank
9. RestKNNStatsHandlerIT.testModelIndexHealthMetricsStats
10. RestSearchModelHandlerIT.testNotSupportedParams
11. RestSearchModelHandlerIT.testNoModelExists
12. RestSearchModelHandlerIT.testSizeValidationFailsInvalidSize
13. RestSearchModelHandlerIT.testSearchModelExists
14. RestSearchModelHandlerIT.testSearchModelWithoutSource
15. RestSearchModelHandlerIT.testSearchModelWithSourceFilteringIncludes
16. RestSearchModelHandlerIT.testSearchModelWithSourceFilteringExcludes

KNNRestTestCase.systemIndexExists

1. RestKNNStatsHandlerIT.testModelIndexHealthMetricsStats

MODEL_INDEX_NAME

1. RestDeleteModelHandlerIT.testTrainingDeletedModel
2. RestKNNStatsHandlerIT.testModelIndexHealthMetricsStats

Solution

For the given tests above, refactor to not directly access the system index. If it does not make sense to refactor, remove test and test functionality through unit tests