If you specify JVM args to run an nsk/jdb test with, the test will not pass these args on to the jdb process or to the debuggee launched by jdb. The jdb arguments are setup in nsk.share.jdb.Launcher.makeJdbCmdLine().

JVM arguments can be passed to the jdb process by prefixing them with -J. JVM arguments can be passed to the debuggee process by prefixing them with -R. This is a new feature just added by JDK-8282691.

It looks like similar support can be added to the com/sun/jdi tests, which also use jdb. See com/sun/jdi/lib/JdbTest.setup(). The difference here is that it does not rely on jdb to launch the debuggee, but instead the test launches the debuggee. setup() launches both of the processes, and it looks like it could be modified to add the JVM options to both processes.