update test isolation pattern

[davidism]

• join_transaction_mode="create_savepoint" tells the SQLAlchemy session to use nested savepoints. The docs mention tests as one reason to use this: https://docs.sqlalchemy.org/en/20/orm/session_api.html#sqlalchemy.orm.Session.params.join_transaction_mode
• engine.connect is patched to return the existing connection.
• connection.close is patched to do nothing.
• connection.begin is patched to call begin_nested instead, since it's already in a transaction for the test.

After experimenting a bunch with a debugger, these were the minimal additional changes I found were needed. I'll look into adding a function to manage this instead of needing to copy from the docs.

closes #19, closes #20

[davidism]

#32 adds a context manager to do this instead of needing to copy all that code from the docs. It will be part of 0.2.