flattened testing module structure

[jorenham]

test_utils::xsref => testing

[Copilot]

Pull Request Overview

This PR flattens the testing module structure by moving the testing utilities from the nested test_utils::xsref module to a top-level testing module.

• Relocates testing infrastructure from test_utils::xsref to testing module
• Updates all test imports to use crate::testing instead of crate::xsref
• Removes the old test_utils module containing the wrapped xsref submodule

Reviewed Changes

Copilot reviewed 33 out of 33 changed files in this pull request and generated no comments.

File	Description
src/lib.rs	Updates module declaration to use new testing module
src/testing.rs	New testing module with utilities moved from test_utils::xsref
src/test_utils.rs	Removes old testing module file
Multiple test files	Updates import statements to use crate::testing

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