Testing a subclass when superclass uses a private method

I have an abstract class Foo that contains a public method abc() and a private method xyz() used in abc() that generates a random string. What abc() returns depends on the return value of xyz().

I also have a subclass Bar that extends Foo.

I would like to test Bar using abc(). However, since I don't know the result of xyz(), I don't know what abc() will return.

The code looks something like this:

public abstract class Foo {
    
    public String abc() {
        *xyz() called here*
    }

    private String xyz() {
        *Generates a random string*
    }

}

public class Bar extends Foo {
}

I tried stubbing or mocking but it doesn't work. I presume that the design of Foo should be changed but am not sure how. I use Spock. Any advice?