test: prevent false-positive Java test runner results

[wenyt (wenytang-ms)]

Summary

• add deterministic coverage for Java test/subject navigation
• make the Java Test Runner plan require proof that the JUnit test actually executed
• remove the permissive path that treated No tests found as a successful run

Verification

• navigation plan: all platforms passed in https://github.com/microsoft/vscode-java-pack/actions/runs/34314118380
• strict runner plan: Windows and Ubuntu passed; macOS was correctly blocked because the execution marker was not produced in https://github.com/microsoft/vscode-java-pack/actions/runs/34315582515