The affected tests are:

com/sun/jdi/cds/CDSBreakpointTest.java
com/sun/jdi/cds/CDSDeleteAllBkptsTest.java
com/sun/jdi/cds/CDSFieldWatchpoints.java

I first ran these tests in JTREG with -compilejdk set to JDK8. It tried compiling CDSBreakpointTest.java with some source path settings to include jdk/testlibrary as well. So Javac will implicitly build jdk/testlibrary/ProcessTool.java, even though it's not explicitly given in the command line.

However, ProcessTools.java failed to compile because it calls Process.getPid() which exists only on JDK9.
        ^
C:\\iklam\\jdk9_rt\\jdk\\test\\lib\\testlibrary\\jdk\\testlibrary\\ProcessTools.java:520: error: cannot find symbol
            return p.getPid();
                    ^
Nevertheless, javac has already created CDSBreakpointTest.class and decided to leave it in the file system.

Now, I ran JTREG again with -compilejdk set to JDK9. Because CDSBreakpointTest.class already exists, it not compiled again. When the test is executed, it gets this error:

----------System.err:(20/1162)----------
java.lang.NoClassDefFoundError: jdk/testlibrary/ProcessTools
    at CDSJDITest.runTest(CDSJDITest.java:71)
    at CDSBreakpointTest.main(CDSBreakpointTest.java:55)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:502)
    at com.sun.javatest.regtest.agent.MainWrapper$MainThread.run(MainWrapper.java:92)
    at java.lang.Thread.run(Thread.java:745)

If @build jdk.testlibrary.* was specified in CDSBreakpointTest.java, then the second execution of JTREG would have tried to make sure all classes in jdk/testlibrary/*.java are compiled, so ProcessTools.class would have been available at run time.

So the morale of the story is (besides "why is JTREG so dumb!") is you should always add @build <library_path>.* "just in case".

Also, these tests have the tag "@library ..". I am not sure how this should be handled.