./mtr --ps-protocol ignores transaction rollback and error during SELECT

MDEV-31002 modified something in the client/mysqltest.cc test driver, to execute SELECT statements a second time in ./mtr --ps-protocol mode. This logic fails to detect that a SELECT in a multi-statement transaction returned an error and that the transaction was aborted. In such a scenario, re-issuing the SELECT statement would implicitly start a new transaction, which would lead to incorrect results.

I see that the change set that implemented MDEV-31002 added --disable_ps2_protocol to a large number of tests. At least some of those cases seem to match this scenario. I found this bug while extending the test case innodb.lock_isolation with a test of MDEV-33802.