Isolate Gradle TestKit fixtures

[iuliiasobolevska]

Summary

• copy Gradle TestKit fixtures into a fresh JUnit temporary directory for every test
• give each test a separate Gradle TestKit home to prevent concurrent cache lock contention
• run nested builds out of process and keep their output isolated from parallel tests
• keep configuration-cache round trips within the same isolated project
• assert after each test that checked-in fixture directories contain no .gradle or build state

Performance

An isolated CodegenGradlePluginTest run with --rerun-tasks --profile completed in 8.951 seconds, compared with 10.382 seconds on master—a 13.8% improvement across all 19 tests.

Verification

• forced four-way parallel execution of all 19 CodegenGradlePluginTest tests
• ./gradlew --info check publishToMavenLocal --warning-mode all
• checked-in src/test/resources contains no generated .gradle or build directories after the suite