Running unit tests with Spock framework

I am using:

• Android Studio 2.1.3
• Gradle 2.14.1 (I tried with 2.14 also)
• OpenJDK version "1.8.0_91"

I want to write some Unit tests with Groovy and Spock for sample Android application.

I have already read about RoboSpock.

When I am trying to run simple test:

package a.b.regex

import spock.lang.Specification

class TestSum extends Specification {


    def "test adding some numbers"() {

        when:
        def a = 5 + 4
        then:
        a = 9

    }
}

I have an error:

Process finished with exit code 1
Class not found: "a.b.regex.TestSum"Empty test suite.

Configurations that I used:

1)

buildscript {
    dependencies {
        classpath 'com.android.tools.build:gradle:2.1.3'
        classpath 'org.codehaus.groovy:gradle-groovy-android-plugin:0.3.6'
    }
}

apply plugin: 'groovyx.grooid.groovy-android'
...
dependencies {
    testCompile 'org.robospock:robospock:1.0.0'
}

2)

buildscript {
    dependencies {
        classpath 'com.android.tools.build:gradle:2.1.3'
        classpath 'org.codehaus.groovy:groovy-android-gradle-plugin:1.0.0'
    }
}

apply plugin: 'groovyx.android'
dependencies {
    testCompile "org.codehaus.groovy:groovy-all:2.4.1"
    testCompile "org.spockframework:spock-core:1.0-groovy-2.4"
    testCompile 'org.codehaus.groovy:groovy:2.4.6:grooid'
}

From the console no tests are run at all. With testing Java application I have no problem.