Method JDKToolLauncher.create doesn't works as expected. It should have create tool using compiler.jdk while it tries to use test.jdk first.
See method doc:
    /**
     * Creates a new JDKToolLauncher for the specified tool. Using tools path
     * from the compiler JDK.
     *
     * @param tool
     * The name of the tool
     * @return A new JDKToolLauncher
     */
    public static JDKToolLauncher create(String tool) {

There is no difference when compiler.jdk is not set. The compiler jdk is the same as tested jdk. Also when the tested jdk doesn't contains tool then it is used from compiled.jdk.
 However there is an issue when tests are executed with jcov. The jcov instrumentation requires using additional heap settings (-Xms64m -Xmx4g).

Tools executed without them fails when running with jcov. Also if tool is not really tested but just used to run test there is no intention to add it's coverage.