refactor: use protected JUnit launch configuration API

[wenytang-ms]

Summary

• replace reflective access to JDT's JUnit launch configuration with the protected API added by Expose JUnit VM runner configuration to subclasses eclipse-jdt/eclipse.jdt.ui#3079
• require org.eclipse.jdt.junit.core 3.15.0 so incompatible Java language server versions fail dependency resolution explicitly

Follow-up to #1885.

Draft status

Keep this PR in draft until the latest redhat.java pre-release packages a JDT LS build containing org.eclipse.jdt.junit.core 3.15.0.

Current published JDT LS snapshot 1.61.0-202607102111 still contains org.eclipse.jdt.junit.core 3.14.300, so Java bundle resolution is expected to fail for now.

• JDT LS snapshot repository contains org.eclipse.jdt.junit.core 3.15.0
• latest redhat.java pre-release packages that JDT LS snapshot
• Java plugin verification passes against the updated target
• mark this PR ready for review

Change size

• 2 files
• 3 insertions, 11 deletions

Validation

mvn -f java-extension\pom.xml -pl com.microsoft.java.test.target,com.microsoft.java.test.plugin,com.microsoft.java.test.plugin.test -am verify

Currently blocked as expected by:

Missing requirement: com.microsoft.java.test.plugin 0.43.1 requires 'osgi.bundle; org.eclipse.jdt.junit.core [3.15.0,4.0.0)' but it could not be found