We know that junit-4.12.jar is a dependency of jtreg graal unit test. In GraalUnitTestLauncher.java, we put this file path into graalJarsCP and concat it to current classpath. But existing classpath may contain another version of junit with which the jtreg tool is built. (According to the "Building jtreg" webpage[1], the recommended version is currently JUnit 4.10)

In order to fix the incompatibility issue of different junit versions, the graalJarsCP should be put before when generating the new classpath string.

[1] https://openjdk.java.net/jtreg/build.html