The system property java.class.path should be considered read-only and not set.
It is specified as being read once at startup.

It appeas to be set during the compile step:

src/share/classes/com/sun/javatest/regtest/agent/ActionHelper.java: System.setProperty("java.class.path", (String) sysProps.get("java.class.path"));
src/share/classes/com/sun/javatest/regtest/agent/CompileActionHelper.java: SearchPath cp = new SearchPath(value, System.getProperty("java.class.path"));
src/share/classes/com/sun/javatest/regtest/agent/CompileActionHelper.java: p.put("java.class.path", cp.toString());
src/share/classes/com/sun/javatest/regtest/agent/MainActionHelper.java: SearchPath cp = new SearchPath(value, System.getProperty("java.class.path"));
src/share/classes/com/sun/javatest/regtest/agent/MainActionHelper.java: p.put("java.class.path", cp.toString());
src/share/classes/com/sun/javatest/regtest/tool/Help.java: