Consider the following command:
    javac -Xmodule:m -p mods -cp lib/junit-4.12.jar test/m/p/MyTest.java

which compiles a test "as if" part of module m (module m has already been compiled to mods/m).

module m is very simple:

src/m/module-info.java:

module m {
    exports p;
}

src/m/p/Main.java:

package p;
public class Main { }


test/m/p/MyTest.java is:

package p;
import org.junit.Test;
import static org.junit.Assert.*;
public class MyTest {
    @Test
    public void testSomething() {
assertTrue(true);
    }
}

Note that the test has references to JUnit tests that are on the class path.

It's not clear how this test compiled successful. It appears that when compiling with -Xmodule that the module is made to read the unnamed module. I can't otherwise explain it.