Remove the direct access of system indices in integration test cases

[weicongs-amazon]

From es 7.10, we will see this warning message: "this request accesses system indices: [.opendistro-anomaly-detectors], but in a future major version, direct access to system indices will be prevented by default"" when one system index is directly accessed.
We need refactor our integration test cases to remove these direct access before next ES major version release

[kaituo]

ref: elastic/elasticsearch#60945

It seems they enforce this through IndexNameExpressionResolver.checkSystemIndexAccess on the rest layer. The fix is to not to directly access them on the rest layer, but on transport layer.