session: add test case for retrying union statement

[djshow832]

What problem does this PR solve?

Add test case for #15050.

What is changed and how it works?

Just add some test cases to test retrying UNION and SHOW statements.

Check List

Tests

• Unit test
• Manual test

I did the same as #15050 and the log shows UNION statement is not retried.

[2020/03/01 18:12:15.574 +08:00] [WARN] [session.go:463] [sql] [conn=1] [label=general] [error="[kv:9007]Write conflict, txnStartTS=414989028796923904, conflictStartTS=414989031736606720, conflictCommitTS=414989031736606721, key={tableID=45, handle=120001} primary=[]byte(nil) [try again later]"] [txn="Txn{state=invalid}"]
[2020/03/01 18:12:15.574 +08:00] [WARN] [session.go:655] [retrying] [conn=1] [schemaVersion=36] [retryCnt=0] [queryNum=0] [sql=begin]
[2020/03/01 18:12:15.575 +08:00] [INFO] [2pc.go:1074] ["2PC clean up done"] [conn=1] [txnStartTS=414989028796923904]
[2020/03/01 18:12:15.575 +08:00] [WARN] [session.go:655] [retrying] [conn=1] [schemaVersion=36] [retryCnt=0] [queryNum=1] [sql="update test set id=id+1"]
[2020/03/01 18:12:15.575 +08:00] [WARN] [session.go:655] [retrying] [conn=1] [schemaVersion=36] [retryCnt=0] [queryNum=2] [sql=commit]

Code changes

N/A

Side effects

N/A

Related changes

N/A

Release note ( This release note applies to the parser PR pingcap/parser#755)

• Fix the bug that goroutine leaks when retrying a transaction which contains UNION statement.

[bb7133]

LGTM

[djshow832]

/run-unit-test

[lysu]

LGTM

[djshow832]

/run-unit-test

[bb7133]

/run-all-tests