Share test kits in a new source set

[Goooler]

Addresses #1081.

[Copilot]

Pull Request Overview

This PR reorganizes test utilities by creating a new testKit source set that contains shared test utility code. The main purpose is to consolidate common test functionality that was previously spread across different packages, making it more reusable and better organized.

• Moves utilities from com.github.jengelman.gradle.plugins.shadow.util and com.github.jengelman.gradle.plugins.shadow.internal to a new testkit package
• Creates dedicated test kit classes for JAR manipulation, resource loading, and test utilities
• Updates all test files to import utilities from the new testkit package instead of the old locations

Reviewed Changes

Copilot reviewed 33 out of 36 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/testKit/kotlin/com/github/jengelman/gradle/plugins/shadow/testkit/Resource.kt	New utility functions for loading test resources as streams and paths
src/testKit/kotlin/com/github/jengelman/gradle/plugins/shadow/testkit/JarPath.kt	Moved JAR manipulation utilities from util package with package rename
src/testKit/kotlin/com/github/jengelman/gradle/plugins/shadow/testkit/Utils.kt	Moved test utilities from util package with package rename
Multiple test files	Updated imports to use new testkit package instead of old util and internal packages
src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/internal/Utils.kt	Removed resource loading functions that were moved to testkit

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