Gradle multi-project build where project2's test classes depend on project1's test classes

gistfile1.txt

.
├── build.gradle
├── project1
│   ├── build.gradle
│   └── src
│       └── test
│           └── java
│               └── sample
│                   └── Foo.java
├── project2
│   ├── build.gradle
│   └── src
│       └── test
│           └── java
│               └── sample
│                   └── BarTest.java
└── settings.gradle

== BarTest.java
package sample;

import org.junit.Test;
import static org.junit.Assert.*;

public class BarTest {
    @Test
    public void doit() {
        assertNotNull(new Foo());
    }
}

== build.gradle (project1)

task testJar(type: Jar) {
    classifier 'test'
    from sourceSets.test.output
}

== build.gradle (project2)

dependencies {
    compile project(':project1')
    testCompile files(project(':project1').testJar.archivePath)
}

What about: testCompile project(':project1').sourceSets.test.output

I'd actually add a configuration to project1 (e.g. tests) and then register the JAR task as outgoing artifact.

configurations {
    tests
}

artifacts {
    tests testJar
}

Then you reference it in project2 like this:

dependencies {
    testCompile project(path: ':project1', configuration: 'tests')
}

First I think it's a very bad idea to design a project which depends on test code of another project.
Now that I've said it, I've migrated a large project from Maven to Gradle in which this bad idea was implemented and I've found that @bmuschko solution was the right way because it accounts for transitive dependencies while @akhikhl solution doesn't.