Trying to add a path to classpath through gradle or some other way

Question

I have a gradle/SpringBoot project. I'm trying to run some junit tests that use 3rd party annotations to inject classes. The injected classes are creating a text file (call it 'foo.txt') under $(PROJECT_DIR)/app/build/resources/test. However the same injected class then uses a classLoader.getReource("foo.txt") to try to find the file to try to process it. The classLoader looks like it is searching for foo.text under $(PROJECT_DIR)/build/resources/test and since it can't find it there the 3rd party class throws an exception.

So I somehow either need to make classLoader.getResource() called from the 3rd party class search in $(PROJECT_DIR)/app/build/resources/test for foo.txt or make the 3rd party class create foo.txt in $(PROJECT_DIR)/build/resources/test.

I was thinking the way to resolve this might be to add $(PROJECT_DIR)/app/build/resources/test to the classpath in gradle for the test task. But I don't know how to add a new path to the classpath through gradle. I searched all over google but don't see any examples.

Also, If there is a better solution I'm open to that too, but I have limitations in how I can try to resolve this i.e. assume I can't change the 3rd party app for now, and can't refactor project structure of the project I am creating tests in.

Update: In case it is helpful here is what the test/s looks like:

@ExtendWith({3rdPartyClassResolver.class})
public class 3rdPartyTest {
      
  //the actual failing test       
  @Test
  @3rdPartyClass(
    storageLocation = "https://storage-location.us/",
    application = "app1",
    version = "latest",
    requestor = "app2")
  public void testing3rdPartyClass(Map<3rdPartyClass, List<Event>> map) {
    System.out.println(); //I don't make it to this point
  }
  //this is what the 3rd party class is trying to do.
  //I'm able to reproduce it using this test. it fails the same way.
  @Test
  public void writeToFile() throws Exception {
    Path pathnio = Paths.get("build/resources/test/foo.txt");
    Files.write(pathnio, Collections.singleton("some text"));
    ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
    URL resource = classLoader.getResource("foo.txt");
    assertNotNull(resource); //this assert fails
  }
}