fix: support extra_lua_path in test framework

[mikyll]

This PR was "forked" from #12759 due to inactivity 👀

Description

This PR adds support for setting custom Lua paths (extra_lua_path and extra_lua_cpath) in test files, aligning the test framework behavior with APISIX runtime configuration.

What Problem Solves

Currently, you are not able to pass an extra Lua path to load custom plugins in APISIX testing framework. That makes it impossible to test custom plugins, without editing APISIX.pm or via other workarounds.

Suppose we have a CI/CD pipeline to release our APISIX instance:

• APISIX container with test-nginx framework
• CI step to test custom plugins
• mount custom plugins volume into the container
• run tests

Currently, the test framework overwrites the value of extra_lua_path of APISIX configuration. As a consequence, APISIX does never pick up the path of your custom plugins and cannot load them, making the tests fail.

This change extends the test framework preprocessor in t/APISIX.pm to extract and prepend custom Lua paths from block definitions or YAML configuration.

Two methods:

1. Block definitions (preferred):

   --- extra_lua_path: /custom/path/?.lua
   --- extra_lua_cpath: /custom/path/?.so
   

2. Automatic parsing from config.yaml:

   apisix:
     extra_lua_path: "/custom/path/?.lua"
     extra_lua_cpath: "/custom/path/?.so"

Implementation

• Prepends custom paths to lua_package_path and lua_package_cpath (matching runtime behavior)
• Block definitions take precedence when both methods are used
• Enables testing custom plugins without modifying core APISIX paths

Example

File t/plugin/my-custom-plugin.t:

=== TEST 1: Test title
Test description
--- extra_lua_path: /path/to/my-custom-plugin/?.lua
--- config
[...]

Which issue(s) this PR fixes:

Fixes #12389

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

Test Coverage:
Added comprehensive test suite in t/admin/extra-lua-path.t covering:

• Path addition via block definitions
• YAML configuration parsing
• Simultaneous lua_path and lua_cpath configuration
• Correct path prepending behavior
• Precedence rules between configuration methods

Backward Compatibility:
This change is fully backward compatible. It only adds new optional block definitions (extra_lua_path, extra_lua_cpath) and parsing logic for existing extra_yaml_config. All existing tests continue to work without modification.