Add tests for the existing functionality

[ilevkivskyi]

We already support descriptor-like behavior and type inference for Columns. We need tests for this so that it will not regress.

Depends on #2.

[ilevkivskyi]

We should add tests for at least most used code patterns:

• Create table
• Create model
• Instantiate model
• Make a query
• Iterate and access attributes on results

Later we can depend additional tests specifically for each feature we want to support by the plugin.

[tapaswenipathak]

Hi @ilevkivskyi: I am adding few tests, let me know if I should add more items/cases like that.