RFR(trivial): 8223054: [TESTBUG] Put graalJarsCP before existing classpath in GraalUnitTestLauncher

[Pengfei Li (Arm Technology China)]

Hi,

Please help review this trivial change on GraalUnitTestLauncher.

Webrev: http://cr.openjdk.java.net/~pli/rfr/8223054/webrev.00/
JBS: https://bugs.openjdk.java.net/browse/JDK-8223054

Current graal unit test in jtreg requires junit-4.12.jar as a dependency. In GraalUnitTestLauncher.java, we put the path of this file into graalJarsCP and concat it with existing classpath. But existing classpath may contain another version of junit with which the jtreg tool is built. (According to OpenJDK "Building jtreg" webpage[1], the recommended version of Junit to build jtreg is junit-4.10).

In this patch, graalJarsCP is put before existing classpath returned by System.getProperty() when generating the new classpath string to avoid incompatibility issues. Jteg graal unit test cases passed after this change.

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

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Thanks,
Pengfei