Merge `extra_init_by_lua_start ` and `extra_init_by_lua ` directive in test cases

Description

Contributor guide

Tech stack

None

Domain

testing

Issue type

refactor

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

2

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

1-3 hours

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

stale

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

Lua basicsAPISIX testing conventions

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

70

Research direction

Review the discussion in https://github.com/apache/apisix/discussions/7988 for context. Identify all test files that contain both `extra init by lua start` and `extra init by lua` directives, and merge them into a single directive where appropriate. Ensure the change maintains test coverage.