Gradle dependency for tests only

I have a java project A and B.

For compile there is no dependency from A to B.

For testing, tests of A have a dependency to B.

Adding this to build.gradle of project A did not solve it:

    dependencies {
        ...
        testCompile project(':B')
    }

Note: What I am trying to accomplish here is to write a test for a class in A(say ClassA) that requires an interface IX. I want to test ClassA with an implementation in project B(say ClassB implements IX), so actually this is kinda integration test.

I would appreciate any pointers for testing two(or more) projects that have no compile dependency but probable runtime dependency(for Java,Eclipse,Gradle).