[Draft] VTGate: Update txn_rollback_shutdown_test.go to actually assert transaction rollback

[brendar]

The original test does not actually assert that the transaction was rolled back. It was just asserting that a new transaction can't see rows written by the other open transaction, which is expected behavior at the default isolation level.

This test now fails with

Error:      	Received unexpected error:
                target: ks.0.primary: vttablet: Statement aborted because lock(s) could not be acquired immediately and NOWAIT is set. (errno 3572) (sqlstate HY000) (CallerID: userData1): Sql: "select * from buffer where id = :id /* INT64 */ for update nowait", BindVars: {#maxLimit: "type:INT64 value:\"10001\""id: "type:INT64 value:\"3\""} (errno 3572) (sqlstate HY000) during query: select * from buffer where id = 3 for update nowait
Test:       	TestTransactionRollBackWhenShutDown
Messages:   	for query: select * from buffer where id = 3 for update nowait

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.49%. Comparing base (83b37b8) to head (9ef4340).
Report is 116 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16839      +/-   ##
==========================================
- Coverage   69.54%   69.49%   -0.05%     
==========================================
  Files        1569     1569              
  Lines      202555   202633      +78     
==========================================
- Hits       140865   140829      -36     
- Misses      61690    61804     +114     

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