add funtional test

src/test/kotlin/CoverallsJacocoPluginTest.kt

@@ -1,13 +1,18 @@
 package org.gradle.plugin.coveralls.jacoco
 
+import io.mockk.every
 import io.mockk.mockk
 import io.mockk.verify
 import org.gradle.api.Action
 import org.gradle.api.Project
 import org.gradle.api.Task
+import org.gradle.api.internal.tasks.DefaultSourceSetContainer
 import org.gradle.api.tasks.SourceSet
+import org.gradle.api.tasks.SourceSetContainer
+import org.gradle.testfixtures.ProjectBuilder
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Test
+import java.io.File
 
 internal class CoverallsJacocoPluginTest {
     @Test
@@ -23,6 +28,20 @@ internal class CoverallsJacocoPluginTest {
         }
     }
 
+    @Test
+    fun `CoverallsJacocoPlugin runs on task execute`() {
+        val sourceSetContainer = mockk<SourceSetContainer>()
+        every { sourceSetContainer.getByName("main").allJava.srcDirs } returns emptySet()
+        val project = ProjectBuilder.builder().withProjectDir(File(".")).build()
+        project.extensions.add("sourceSets", sourceSetContainer)
+
+        val plugin = CoverallsJacocoPlugin()
+        plugin.apply(project)
+
+        val task = project.tasks.getByName("coverallsJacoco")
+        task.actions.forEach { it.execute(task) }
+    }
+
     @Test
     fun `CoverallsJacocoPluginExtension has meaningful defaults`() {
         val extension = CoverallsJacocoPluginExtension()