fix flaky test in DefinedIndexesIntegrationTests

[CharlesZKQ]

In org/springframework/data/gemfire/config/support/DefinedIndexesIntegrationTests.java, the test indexesCreated() is flaky due to the non-deterministic property of getDeclaredMethods() (please refer document), which is used when creating the list definedIndexNames. I fixed it by sort both the definedIndexNames (actual) and expectedDefinedIndexNames in alphabetical order, so that it is guaranteed that these two are equal to each other.