add tests for (and maybe fix) transaction statements `BEGIN;`...`COMMIT;`

[psteinroe]

Bug report

Describe the bug

I am not sure yet whether this is a bug, but we should add tests for transaction statements to see how they are parsed by lipg_query. They should be parsed as a single statement. We may have to adapt the source_parser regex for that.

Expected behavior

Transactions are beneath a single ast node, and a nested cst node.