In jdk/make/RunTests.gmk, it makes use of jtreg's `-testjdk:dir` option for specifying the testing JDK for running jtreg tests. It's also useful to be able to specify the JDK to compile code, with the `-compilejdk:dir` option.

Normally, the testing JDK contains the `javac`, which can be used to compile the test code. With running on the static JDK build (built by `static-jdk-image`), the `static-jdk` image does not contain `javac` or other tools. We want to use an alternative JDK with the tools for compiling tests. In that case, specifying the `-compilejdk:dir` option is needed.