test: fix user without process privilege can access cluster_tidb_trx

[AilinKid]

What problem does this PR solve?

Issue Number: close #26127

What is changed and how it works?

*: What's Changed: add the privileges check when read the memory table

Check List

Tests

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

Documentation

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

Release note

• test: fix user without process privilege can access cluster_tidb_trx

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[mjonss]

I think you also fixed:
CLUSTER_SYSTEMINFO, CLUSTER_HARDWARE and CLUSTER_LOAD, or is that just from a previous patch?

[MyonKeminta]

In our design TIDB_TRX and CLUSTER_TIDB_TRX shows all information to users with PROCESS privilege, but it only shows transactions executed by the current user if the user doesn't have PROCESS privilege. Is this design improper or you misunderstood its behavior?

[AilinKid]

In our design TIDB_TRX and CLUSTER_TIDB_TRX shows all information to users with PROCESS privilege, but it only shows transactions executed by the current user if the user doesn't have PROCESS privilege. Is this design improper or you misunderstood its behavior?

Misunderstood before. Maybe the designer see a user without PROCESS privilege get access to this even though it gets empty result set.

[MyonKeminta]

This actually referenced the processlist table:

If you have the PROCESS privilege, you can see all threads, even those belonging to other users. Otherwise (without the PROCESS privilege), nonanonymous users have access to information about their own threads but not threads for other users, and anonymous users have no access to thread information.

It's a problem that this behavior seems to lack of test coverage. cc @longfangsong
And I just notice that the privilege requirements to the system tables are not well explained in our documentation.