sysvar: fix inconsistency in setting password validation sysvars

[CbcWestwolf]

What problem does this PR solve?

Issue Number: ref #38928

Also merge the PR in tidb-test

Problem Summary:

Since sysvars for password validation satisfy length >= number_count + special_char_count + 2 * mixed_case_count, altering the latter 3 sysvars may also alter length

What is changed and how it works?

Check List

Tests

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

• bb7133
• hawkingrei

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[CbcWestwolf]

/run-mysql-test tidb-test=pr/2031

[bb7133]

LGTM

[bb7133]

/merge

[ti-chi-bot]

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

Commit hash: e55a478

[CbcWestwolf]

/run-mysql-test tidb-test=pr/2031

[sre-bot]

TiDB MergeCI notify

🔴 Bad News! New failing [1] after this pr merged.
These new failed integration tests seem to be caused by the current PR, please try to fix these new failed integration tests, thanks!

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci-tidb/integration-ddl-test	🟥 failed 1, success 5, total 6	4 min 13 sec	New failing
idc-jenkins-ci/integration-cdc-test	🔴 failed 2, success 38, total 40	24 min	Existing failure
idc-jenkins-ci-tidb/integration-common-test	🔴 failed 2, success 15, total 17	11 min	Existing failure
idc-jenkins-ci-tidb/common-test	🔴 failed 1, success 10, total 11	9 min 42 sec	Existing failure
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	5 min 53 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	4 min 53 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-1	🟢 all 26 tests passed	4 min 2 sec	Existing passed
idc-jenkins-ci-tidb/mybatis-test	🟢 all 1 tests passed	3 min 52 sec	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	2 min 52 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #39845.