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 -PswiftJavaJdk=23 :Samples:SwiftJavaExtractFFMSampleApp:test :Samples:SwiftAndJavaJarFFMSampleLib:test --no-daemon

The relevant test suites pass with zero failures, errors, or skipped tests.

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?

[ktoso]

Thanks for the contribution, a single PR is good, the sample apps serve as "end to end tests" really, so it is good to update them along with fixes