backend: add tests for prepared statements

[djshow832]

What problem does this PR solve?

Issue Number: close #32

Problem Summary:
Add tests for forwarding prepared statements and check the status.

What is changed and how it works:

• Add tests for forwarding prepared statements to see if they will hang.
• Verify that all the response packets are forwarded to the client for all commands by checking the sequence.
• Add tests for querying directly from the backend, which is currently used by show session_states.
• Add tests for checking if the session is redirectable after executing prepared statements.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Notable changes

• Has configuration change
• Has HTTP API interfaces change (Don't forget to add the declarative for API)
• Has weirctl change
• Other user behavior changes

Release note

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

None

[xhebox]

Rest LGTM