Update MasterSlaveConnectionTest.php

the test testKeepSlaveBeginTransactionStaysOnMaster does not begin a transaction, I have put a begin transaction, and now it fails,

the fix is here, if it makes sense
https://github.com/ananda-agrawal/dbal/commit/eb9262afb3955e4974d5a713b495a544b66a5cf7
[DBAL-717] Fix bug in MasterSlaveConnection with keepSlave option and switch back after transaction.