Support compilation in Proc test utility

The Proc test utility is able to launch a class in a separate process, but the class must be compiled first. In most cases, the class is either the launcher class itself (where it's already compiled) or it can be compiled explicitly with the @build jtreg tag. Still, in some cases, the test can only be built with a special classpath or other options. It will be nice if Proc support compilation as well.