Fix more flaky integration tests

(cherry picked from commit 1d179dac)

This PR formats all the `*.py` files found under the `tests/integration`
folder. It also reorders the imports and cleans up a bunch of unused
imports.

The formatting also takes care of other things like wrapping lines and
fixing spaces and indents such that the tests look more readable.

* Move some code outside dbms/src folder
* Fix paths

This reverts commit 24bdad37, reversing
changes made to dad4f03a.