tests: Fix unstable schema test

[JaySon-Huang]

What problem does this PR solve?

Issue Number: close #8968, close #8911

Problem Summary:
#8955 introduced a check on the cache gc_safepoint is not updated, but the background task of DDL sync make the ut unstable

What is changed and how it works?

• Disable the background task when profiles.default.ddl_sync_interval_seconds == 0
• Refine the code in TiDBSchemaManager.h with comments on locking schema_syncers_mutex
• Refine the code in SchemaSyncTest to use temporary instance instead of the instance in global_context

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

None

[JaySon-Huang]

/run-all-tests

[JaySon-Huang]

/run-all-tests

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JinheLin, Lloyd-Pottiger

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

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• OWNERS [JinheLin,Lloyd-Pottiger]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-04-21 04:17:38.123285802 +0000 UTC m=+743677.234332243: ☑️ agreed by Lloyd-Pottiger.
• 2024-04-22 02:28:36.455938117 +0000 UTC m=+823535.566984566: ☑️ agreed by JinheLin.

[JaySon-Huang]

/cherry-pick release-8.1

[ti-chi-bot]

@JaySon-Huang: new pull request created to branch release-8.1: #8974.

In response to this:

/cherry-pick release-8.1

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.

[JaySon-Huang]

/cherry-pick release-7.5

[ti-chi-bot]

@JaySon-Huang: new pull request created to branch release-7.5: #8975.

In response to this:

/cherry-pick release-7.5

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.