Make IDEA tests special

As described here:
diffplug#200 (comment)

gradle/special-tests.gradle

@@ -6,6 +6,7 @@ def special = [
 	'Buf',
 	'Clang',
 	'gofmt',
+	'idea',
 	'Npm',
 	'Shfmt'
 ]

plugin-gradle/src/test/java/com/diffplug/gradle/spotless/JavaIdeaTest.java

@@ -18,8 +18,9 @@
 import java.io.IOException;
 
 import org.junit.jupiter.api.Test;
+import com.diffplug.spotless.tag.IdeaTest;
 
-
+@IdeaTest
 class JavaIdeaTest extends GradleIntegrationHarness {
 	@Test
 	void idea() throws IOException {

plugin-maven/src/test/java/com/diffplug/spotless/maven/java/IdeaTest.java

@@ -19,6 +19,7 @@
 
 import com.diffplug.spotless.maven.MavenIntegrationHarness;
 
+@com.diffplug.spotless.tag.IdeaTest
 class IdeaTest extends MavenIntegrationHarness {
 	@Test
 	void idea() throws Exception {

testlib/src/main/java/com/diffplug/spotless/tag/IdeaTest.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2021-2024 DiffPlug
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.diffplug.spotless.tag;
+
+import static java.lang.annotation.ElementType.METHOD;
+import static java.lang.annotation.ElementType.TYPE;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+import org.junit.jupiter.api.Tag;
+
+@Target({TYPE, METHOD})
+@Retention(RUNTIME)
+@Tag("idea")
+public @interface IdeaTest {}

testlib/src/test/java/com/diffplug/spotless/java/IdeaStepTest.java

@@ -7,7 +7,9 @@
 import com.diffplug.common.io.Files;
 import com.diffplug.spotless.FormatterStep;
 import com.diffplug.spotless.ResourceHarness;
+import com.diffplug.spotless.tag.IdeaTest;
 
+@IdeaTest
 class IdeaStepTest extends ResourceHarness {
 
 	@Test