Fix custom native test classpath wiring

[jormundur00]

Fixes #702

Spec fit

Issue 702 is compatible with the Gradle native-test contract: a custom native test binary registered
with registerTestBinary should derive classes, resources, runtime classpath, test identifiers, and
JUnit native support from the configured source set and Test task. This PR adds the explicit
custom-test spec point for that behavior and cites it from the Gradle plugin implementation and
functional regression coverage.

Implementation summary

• Adds FS-gradle-native-tests.1.1 for custom native test binaries.
• Changes native test configuration creation to use the selected source set runtime classpath
  configuration instead of always using the default testRuntimeClasspath.
• Adds a functional regression test that checks nativeImageIntegTestClasspath includes the custom
  source set project/JUnit dependencies and plugin-provided junit-platform-native.
• Updates the custom-test sample so it no longer inherits dependencies from the default test
  configurations, avoiding the masking behavior from the original bug.

Validation evidence

• JAVA_HOME=/home/jovan/.sdkman/candidates/java/17.0.12-graal ./gradlew :native-gradle-plugin:functionalTest --tests "org.graalvm.buildtools.gradle.JavaApplicationWithTestsFunctionalTest.custom test image classpath follows custom source set"
• JAVA_HOME=/home/jovan/.sdkman/candidates/java/17.0.12-graal ./gradlew :native-gradle-plugin:functionalTest --tests "org.graalvm.buildtools.gradle.JavaApplicationWithTestsFunctionalTest.can register a custom test image"
• JAVA_HOME=/home/jovan/.sdkman/candidates/java/17.0.12-graal ./gradlew :native-gradle-plugin:test :native-gradle-plugin:inspections
• JAVA_HOME=/home/jovan/.sdkman/candidates/java/17.0.12-graal ./gradlew :native-gradle-plugin:configCacheFunctionalTest --tests "org.graalvm.buildtools.gradle.JavaApplicationWithTestsFunctionalTest.custom test image classpath follows custom source set"
• git diff --check
• grund gradle/FS-gradle-native-tests.1.1 --brief

grund check was also run and failed only on pre-existing outdated grund init blocks in repository
AGENTS.md files. The new spec section resolves directly. Full Gradle functional matrices, full
repository build, and an issue-specific native-image compile/runtime execution were not run locally.

Review readiness

Local aggregate review found no requirements, spec/grund, implementation, or blocking validation
findings. Ready to publish: yes.

Remaining human follow-up

• Review the focused validation scope and decide whether to run broader CI or native-image runtime
  coverage before marking the PR ready.
• Resolve the pre-existing outdated grund init blocks separately if a clean full grund check is
  required for repository maintenance.