Get more tests for BatchParserWrapper and ExecutionEngine

[abist]

Write more tests for the BatchParserWrapper class as well as the ExecutionEngine class. These classes have low code coverage in coveralls right now, and can be definitely be increased.

[coquagli]

Maybe move this to vscode-mssql?

[abist]

@coquagli This is supposed to be added with the BatchParserWrapper itself, so I think putting it here should be fine.

[benrr101]

@coquagli is right, we generally keep track of tasks in vscode-mssql. I don't personally agree with it, but if we have a convention, we should stick to it to avoid confusion.

[abist]

@benrr101 @coquagli Sure, I'll put this in vscode-mssql.