Add currently broken and disabled mixin build test

src/test/groovy/net/fabricmc/loom/BuildUtils.groovy

@@ -155,7 +155,7 @@ static String genModJsonFile() {
 """
 }
 
-static String genModExampleFile() {
+static String genModJavaFile() {
 	"""
 package net.fabricmc.example;
 
@@ -173,3 +173,41 @@ public class ExampleMod implements ModInitializer {
 }
 """
 }
+
+static String genModMixinsJsonFile() {
+	"""
+{
+  "required": true,
+  "package": "net.fabricmc.example.mixin",
+  "compatibilityLevel": "JAVA_8",
+  "mixins": [
+  ],
+  "client": [
+    "ExampleMixin"
+  ],
+  "injectors": {
+    "defaultRequire": 1
+  }
+}
+"""
+}
+
+static String genModMixinsJavaFile() {
+	"""
+package net.fabricmc.example.mixin;
+
+import net.minecraft.client.MinecraftClient;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
+
+@Mixin(MinecraftClient.class)
+public class ExampleMixin {
+	@Inject(at = @At("HEAD"), method = "init()V")
+	private void init(CallbackInfo info) {
+		System.out.println("This line is printed by an example mod mixin!");
+	}
+}
+"""
+}

src/test/groovy/net/fabricmc/loom/MixinBuildFunctionalTest.groovy

@@ -0,0 +1,76 @@
+package net.fabricmc.loom
+
+import org.gradle.testkit.runner.GradleRunner
+import org.junit.Rule
+import org.junit.rules.TemporaryFolder
+import spock.lang.Ignore
+import spock.lang.Specification
+import spock.lang.Unroll
+
+import static net.fabricmc.loom.BuildUtils.*
+import static org.gradle.testkit.runner.TaskOutcome.SUCCESS
+
+/**
+ * Created by Mitchell Skaggs on 6/10/2019.
+ */
+class MixinBuildFunctionalTest extends Specification {
+	@Rule
+	TemporaryFolder testProjectDir = new TemporaryFolder()
+	File settingsFile
+	File buildFile
+	File propsFile
+	File modJsonFile
+	File modJavaFile
+	File modMixinsJsonFile
+	File modMixinsJavaFile
+
+	def setup() {
+		settingsFile = testProjectDir.newFile('settings.gradle')
+		buildFile = testProjectDir.newFile('build.gradle')
+		propsFile = testProjectDir.newFile('gradle.properties')
+
+		testProjectDir.newFolder("src", "main", "resources")
+		modJsonFile = testProjectDir.newFile('src/main/resources/fabric.mod.json')
+		modMixinsJsonFile = testProjectDir.newFile('src/main/resources/modid.mixins.json')
+
+		testProjectDir.newFolder("src", "main", "java", "net", "fabricmc", "example")
+		modJavaFile = testProjectDir.newFile("src/main/java/net/fabricmc/example/ExampleMod.java")
+
+		testProjectDir.newFolder("src", "main", "java", "net", "fabricmc", "example", "mixin")
+		modMixinsJavaFile = testProjectDir.newFile("src/main/java/net/fabricmc/example/mixin/ExampleMixin.java")
+	}
+
+	/**
+	 * Why it's ignored:
+	 * <p>
+	 * When doing mixin annotation processing, a {@link ServiceLoader} is used to find implementations of {@link org.spongepowered.tools.obfuscation.service.IObfuscationService}. The classpath isn't passed to the build properly in this case, causing the Fabric-specific {@link net.fabricmc.loom.mixin.ObfuscationServiceFabric} to not be loaded. This causes the mixin to fail to compile because it doesn't know how to be reobfuscated.
+	 */
+	@Unroll
+	@Ignore("fails because the annotation processor classpath doesn't include fabric-mixin-compile-extensions, so it doesn't know how to remap anything")
+	def "mixin build succeeds using Minecraft #mcVersion"() {
+		given:
+		settingsFile << genSettingsFile("mixin-build-functional-test")
+		propsFile << genPropsFile(mcVersion, yarnVersion, loaderVersion, fabricVersion)
+		buildFile << genBuildFile()
+		modJsonFile << genModJsonFile()
+		modJavaFile << genModJavaFile()
+		modMixinsJsonFile << genModMixinsJsonFile()
+		modMixinsJavaFile << genModMixinsJavaFile()
+
+		when:
+		def result = GradleRunner.create()
+				.withPluginClasspath()
+				.withProjectDir(testProjectDir.root)
+				.withArguments('build')
+				.build()
+
+		then:
+		result.task(":build").outcome == SUCCESS
+
+		where:
+		mcVersion | yarnVersion       | loaderVersion     | fabricVersion
+		'1.14'    | '1.14+build.21'   | '0.4.8+build.155' | '0.3.0+build.184'
+		'1.14.1'  | '1.14.1+build.10' | '0.4.8+build.155' | '0.3.0+build.184'
+		'1.14.2'  | '1.14.2+build.7'  | '0.4.8+build.155' | '0.3.0+build.184'
+	}
+}

src/test/groovy/net/fabricmc/loom/SimpleBuildFunctionalTest.groovy

@@ -40,7 +40,7 @@ class SimpleBuildFunctionalTest extends Specification {
 		propsFile << genPropsFile(mcVersion, yarnVersion, loaderVersion, fabricVersion)
 		buildFile << genBuildFile()
 		modJsonFile << genModJsonFile()
-		modExampleFile << genModExampleFile()
+		modExampleFile << genModJavaFile()
 
 		when:
 		def result = GradleRunner.create()