test: re-enable FFM sample coverage

[Hokila]

Summary

• Re-enable the FFM Runnable upcall integration test in both sample apps.
• Update the JAR FFM sample to use java.lang.Runnable with run().
• Re-enable the FFM instance-property test for getX().
• Correct three UnaryOperator tests that accidentally called the corresponding BinaryOperator methods.

Testing

./gradlew :Samples:SwiftJavaExtractFFMSampleApp:test :Samples:SwiftAndJavaJarFFMSampleLib:test --no-daemon

Tested with JDK 25.0.4.1. Both FFM sample test tasks pass successfully.

Question for maintainers

These changes are related FFM sample-test maintenance across two sample applications. Would you prefer this to remain as one PR, or should it be split into separate PRs per sample/application?

This supersedes #917, which was closed automatically when the source branch was renamed.

[ktoso]

Thanks!