Build.gradle

apply plugin: 'java'
apply plugin: 'com.example.testplugin'

build.gradle

buildscript {
    repositories {
     mavenCentral()
        dependencies {
        }
    }
}

plugins {
    id 'groovy'
    id 'java'
    id 'java-gradle-plugin'
    id 'idea'
}

repositories {
       mavenCentral()
}

group 'com.example'
version '1.0-SNAPSHOT'


sourceCompatibility = 1.8

sourceSets {
    main {
        java { srcDirs = [] }    // no source dirs for the java compiler
        groovy { srcDirs = ["src/main/java", "src/main/groovy"] }  // compile   everything in src/ with groovy
    }
    functionalTest {
        groovy {
            srcDir file('src/functionalTest/groovy')
            outputDir = file('out/functionalTest')
            resources{
                file("src/functionalTest/resources")
            }
        }
        java {
            srcDir file('src/functionalTest/java')
            outputDir = file('out/functionalTest')
            resources{
                file("src/functionalTest/resources")
            }
        }
        resources {
            srcDir file('src/functionalTest/resources')
        }
        
        compileClasspath += sourceSets.main.output + configurations.testRuntime
        runtimeClasspath += output + compileClasspath
    }
}

dependencies {
    compile "org.codehaus.groovy:groovy-all:$groovy_version"
    compile group: 'org.apache.logging.log4j', name: 'log4j-api', version: log4j_version
    compile group: 'org.apache.logging.log4j', name: 'log4j-core', version: log4j_version
    testCompile(
            'org.mockito:mockito-all:1.10.19',
            "org.junit.platform:junit-platform-runner:$junit_platform_version",
            "org.junit.jupiter:junit-jupiter-params:$junit_version",
            'org.spockframework:spock-unitils:1.1-groovy-2.4',
            "net.bytebuddy:byte-buddy:1.8.0",
            "org.objenesis:objenesis:2.6"
    )
    testCompile group: 'org.junit-pioneer', name: 'junit-pioneer', version: junit_pioneer_version
    testCompileOnly(
            'junit:junit:4.12'
    )
    testImplementation(
            "org.junit.jupiter:junit-jupiter-api:$junit_version"
    )
    testRuntimeOnly(
            "org.junit.vintage:junit-vintage-engine:$junit_version",
            "org.junit.jupiter:junit-jupiter-engine:$junit_version",
            "org.junit.platform:junit-platform-launcher:$junit_platform_version"
    )
    functionalTestCompile(
            gradleTestKit(),
            sourceSets.main.output,
            'org.junit-pioneer:junit-pioneer:0.1.2',
            'org.mockito:mockito-all:1.10.19',
            "org.junit.platform:junit-platform-runner:$junit_platform_version",
            "org.junit.jupiter:junit-jupiter-params:$junit_version",
    )

    functionalTestImplementation(
            "org.junit.jupiter:junit-jupiter-api:$junit_version"
    )
    functionalTestRuntimeOnly(
            "org.junit.jupiter:junit-jupiter-engine:$junit_version",
            "org.junit.platform:junit-platform-launcher:$junit_platform_version",
            files("./build/pluginUnderTestMetadata/plugin-under-test-metadata.properties")
    )
}
gradlePlugin {
    plugins {
        testPlugin {
            id = 'com.example.testplugin'
            implementationClass = 'com.example.testplugin.TestPlugin'
        }
    }
    testSourceSets sourceSets.functionalTest
}
task functionalTest(type: Test) {
    group = "verification"
    testClassesDirs = sourceSets.functionalTest.output.classesDirs
    classpath = sourceSets.functionalTest.runtimeClasspath
    outputs.upToDateWhen { false }
    useJUnitPlatform()
}

idea {
    module {
        testSourceDirs += project.sourceSets.functionalTest.java.srcDirs
        testSourceDirs += project.sourceSets.functionalTest.groovy.srcDirs
        testSourceDirs += project.sourceSets.functionalTest.resources.srcDirs
    }
}

TesrtRun.java

import org.gradle.testkit.runner.BuildResult;
import org.gradle.testkit.runner.GradleRunner;
import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Tag;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.junitpioneer.jupiter.TempDirectory;

import java.io.*;
import java.nio.file.Path;

/**
 * @author Kis András Gábor
 * 2018.08.03.
 */
@DisplayName("Testing the plugin")
public class TestRun {

    @Test
    @Tag("functional")
    @ExtendWith(TempDirectory.class)
    @DisplayName("the plugin can be applied and configured")
    void test(@TempDirectory.TempDir Path directory) throws IOException {
        final BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(getClass().getResource("Build.gradle").openStream()));
        final String buildScript = bufferedReader.lines().reduce("", (accumulator, current) -> accumulator + current);
        bufferedReader.close();
        
        final File buildFile = new File(directory + "/build.gradle");
        final FileWriter buildFileWriter = new FileWriter(buildFile);
        buildFileWriter.write(buildScript);
        buildFileWriter.close();

        final BuildResult result = GradleRunner.create()
                .withProjectDir(directory.toFile())
                .withPluginClasspath()
                .withGradleVersion("4.6")
                .build();

        System.out.println("result = " + result.getOutput());
    }
}