Fix Private_Access_Error in Ordered_Multi_Value_Key

[GregoryTravis]

Added a test-only accessor.

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[JaroslavTulach]

Please hide (if it is not possible to remove) test only APIs.

[JaroslavTulach]

Adding the get_java_column_test_only into In_Memory_Test_Helpers is more veiled than having it directly in the In_Memory_Column. Thank you.