backend: add tests for transaction status

[djshow832]

What problem does this PR solve?

Issue Number: close #34

Problem Summary:
Add tests for transaction status.

What is changed and how it works:

• TestTxnStatus: test whether the session is redirect-able when it has an active transaction.
• TestMixPrepAndTxnStatus: test whether the session is redirect-able for mixed prepared statement status and txn status.
• TestHoldRequest: test that the BEGIN statement will be held when the session is waiting for redirection.

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