Gradle Test Dependency

This is a simpler solution that doesn't require an intermediate jar file:

dependencies {
  ...
  testCompile project(':aProject').sourceSets.test.output
}

There's more discussion in this question: Multi-project test dependencies with gradle

You can expose the test classes via a 'tests' configuration and then define a testCompile dependency on that configuration.

I have this block for all java projects, which jars all test code:

task testJar(type: Jar, dependsOn: testClasses) {
    baseName = "test-${project.archivesBaseName}"
    from sourceSets.test.output
}

configurations {
    tests
}

artifacts {
    tests testJar
}

Then when I have test code I want to access between projects I use

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

This is for Java; I'm assuming it should work for groovy as well.

This works for me (Java)

// use test classes from spring-common as dependency to tests of current module
testCompile files(this.project(':spring-common').sourceSets.test.output)
testCompile files(this.project(':spring-common').sourceSets.test.runtimeClasspath)

// filter dublicated dependency for IDEA export
def isClassesDependency(module) {
     (module instanceof org.gradle.plugins.ide.idea.model.ModuleLibrary) && module.classes.iterator()[0].url.toString().contains(rootProject.name)
}

idea {
      module {
          iml.whenMerged { module ->
              module.dependencies.removeAll(module.dependencies.grep{isClassesDependency(it)})
              module.dependencies*.exported = true
          }
      }
  }
.....  
// and somewhere to include test classes 
testRuntime project(":spring-common")