test: do not overwrite the log config in other tests

[tangenta]

What problem does this PR solve?

Issue Number: related to #30871.

Problem Summary: #30871 sets the log level to a value read from the env variable log_level for every test package. However, some of the package overwrite the log config in tests. The log level in tests is out of control:

$ log_level=warn go test -v github.com/pingcap/tidb/server -test.run TestExplainFor -count 1

It is still printing the "INFO" level log:

...
[2021/12/24 19:25:26.188 +08:00] [INFO] [server.go:246] ["server is running MySQL protocol"] [addr=127.0.0.1:0]
[2021/12/24 19:25:26.188 +08:00] [INFO] [http_status.go:87] ["for status and metrics report"] ["listening on addr"=127.0.0.1:0]
[2021/12/24 19:25:26.202 +08:00] [INFO] [session.go:3012] ["CRUCIAL OPERATION"] [conn=5] [schemaVersion=29] [cur_db=] [sql="DROP DATABASE IF EXISTS `explain_for_conn`;"] [user=root@%]
...

What is changed and how it works?

This PR tries to prevent the log-config from being overwritten unexpectedly.

Check List

Tests

• Unit test

• Integration test

• Manual test (add detailed scripts or steps below)

  After this PR, only WARN level log is printed:

  ...
  [2021/12/24 19:31:04.927 +08:00] [WARN] [sysvar_cache.go:53] ["sysvar cache is empty, triggering rebuild"]
  [2021/12/24 19:31:04.932 +08:00] [WARN] [misc.go:454] ["Automatic TLS Certificate creation is disabled"] []
  ...
  --- PASS: TestExplainFor (0.27s)
  PASS
  ok  	github.com/pingcap/tidb/server	0.919s

• 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

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Defined2014
• hawkingrei

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/771fd4ce3992b071b601407baad19351a6c2081f

[tangenta]

/merge

[ti-chi-bot]

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

Commit hash: 7e2f6da

[ti-chi-bot]

@tangenta: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.

[tangenta]

/hold

[tangenta]

/merge cancel because the CI is overload.

[tangenta]

/merge

[ti-chi-bot]

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

Commit hash: cc4b7fb

[tangenta]

/unhold

[hawkingrei]

/run-mysql-test