util/ddl-checker: fix DATA RACE in TestExecute

[tiancaiamao]

What problem does this PR solve?

Issue Number: close #33701

Problem Summary:

What is changed and how it works?

The Exec() result should be closed, otherwise the background coprocessor goroutine is running,
it would DATA RACE with the next Exec().

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

After #33765, you can verify it with

make ut X='run util/ddl-checker TestExecute --race'

Before:

#33752 (comment)

After:

make ut X='run util/ddl-checker TestExecute --race'
tools/bin/ut run util/ddl-checker TestExecute --race || { find $PWD/ -type d | grep -vE "(\.git|tools)" | xargs tools/bin/failpoint-ctl disable; exit 1; }
2022/04/07 16:57:28 maxprocs: Leaving GOMAXPROCS=16: CPU quota undefined
building task finish, count=1, takes=7.317796473s
run all tasks takes 939.7532ms

• [] No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hawkingrei
• ngaut

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/ec3918feb5307e25696c0f97cbe4a47d6d01605e

[Defined2014]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: d6b410c

[tiancaiamao]

Close #33766