Fix flaky PartitionManager test

TestPartitionManager/starting_transaction_failed_due_to_cancelled_context is asserting that context cancellation is taken into account while beginning a transaction. The test is currently flaky as it may be that the TransactionManager finishes initializing before Begin() is called. This then triggers indeterministic behavior in Begin(). Either the transaction begins successfully, or the context cancelation is noticed and it errors out. Fix this flake by preventing the TransactionManager from running in the test so it never initializes and the Begin will time out.