Add test cases for converting SQL with joins to MongoDB

[apavlo]

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We need to write test cases that check whether Sql2Mongo can successfully convert SQL queries that join multiple tables into multiple Operations.

• We should check to make sure that each individual Operation is correct.
• We should check that the Operations is linked together properly with the correct query_group identifier.

We already have a test case for this in unittest_sql2mongo but it is currently commented out. We need to figure out what's wrong with it.