Add UP-TO-DATE tests for changing paths

This test should cover google#557.

src/test/groovy/com/google/protobuf/gradle/ProtobufJavaPluginTest.groovy

@@ -484,6 +484,61 @@ class ProtobufJavaPluginTest extends Specification {
     gradleVersion << GRADLE_VERSIONS
   }
 
+  @Unroll
+  void "test proto generation is not up-to-date on path changes [gradle #gradleVersion]"() {
+    given: "project from testProject"
+    File projectDir = ProtobufPluginTestHelper.projectBuilder('testProject')
+            .copyDirs('testProjectBase', 'testProject')
+            .build()
+
+    when: "protoc path is set and build is invoked"
+    def buildGradleFile = new File(projectDir, "build.gradle")
+    buildGradleFile.append("""
+        configurations {
+          protoc
+        }
+
+        dependencies {
+          protoc "com.google.protobuf:protoc:3.0.0:\$project.osdetector.classifier@exe"
+        }
+
+        protobuf {
+          protoc {
+            path = "\$configurations.protoc.singleFile"
+          }
+        }""")
+    def result = GradleRunner.create()
+            .withProjectDir(projectDir)
+            .withArguments('build', '--stacktrace')
+            .withPluginClasspath()
+            .withGradleVersion(gradleVersion)
+            .forwardStdOutput(new OutputStreamWriter(System.out))
+            .forwardStdError(new OutputStreamWriter(System.err))
+            .withDebug(true)
+            .build()
+
+    then: "it succeeds"
+    result.task(":generateProto").outcome == TaskOutcome.SUCCESS
+
+    when: "protoc path is changed and build runs again"
+    buildGradleFile.text = buildGradleFile.text.replace("com.google.protobuf:protoc:3.0.0", "com.google.protobuf:protoc:3.0.2")
+    result = GradleRunner.create()
+            .withProjectDir(projectDir)
+            .withArguments('build', '--stacktrace')
+            .withPluginClasspath()
+            .withGradleVersion(gradleVersion)
+            .forwardStdOutput(new OutputStreamWriter(System.out))
+            .forwardStdError(new OutputStreamWriter(System.err))
+            .withDebug(true)
+            .build()
+
+    then: "generateProto is not UP_TO_DATE"
+    result.task(":generateProto").outcome == TaskOutcome.SUCCESS
+
+    where:
+    gradleVersion << GRADLE_VERSIONS
+  }
+
   @Unroll
   void "test proto extraction is up-to-date for testProject when changing java sources [gradle #gradleVersion]"() {
     given: "project from testProject"