Add a test that triggers the StableConfigurationCache bug.

Author: nedtwigg

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2020-2021 DiffPlug
+ * Copyright 2020-2022 DiffPlug
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,7 +25,8 @@ public class ConfigurationCacheTest extends GradleIntegrationHarness {
 	@Override
 	protected GradleRunner gradleRunner() throws IOException {
 		setFile("gradle.properties").toContent("org.gradle.unsafe.configuration-cache=true");
-		return super.gradleRunner().withGradleVersion(GradleVersionSupport.CONFIGURATION_CACHE.version);
+		setFile("settings.gradle").toContent("enableFeaturePreview(\"STABLE_CONFIGURATION_CACHE\")");
+		return super.gradleRunner().withGradleVersion(GradleVersionSupport.STABLE_CONFIGURATION_CACHE.version);
 	}
 
 	@Test

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

@@ -43,7 +43,9 @@ public class GradleIntegrationHarness extends ResourceHarness {
 	public enum GradleVersionSupport {
 		JRE_11("5.0"), MINIMUM(SpotlessPlugin.MINIMUM_GRADLE),
 		// technically, this API exists in 6.5, but the flags for it change in 6.6, so we build to that
-		CONFIGURATION_CACHE("6.6");
+		CONFIGURATION_CACHE("6.6"),
+		// https://docs.gradle.org/7.5/userguide/configuration_cache.html#config_cache:stable
+		STABLE_CONFIGURATION_CACHE("7.5");
 
 		final String version;