Reuse testKitDir (#1966)

Author: nedtwigg

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

@@ -119,6 +119,7 @@ protected GradleRunner gradleRunner() throws IOException {
 		return GradleRunner.create()
 				.withGradleVersion(GradleVersionSupport.MINIMUM.version)
 				.withProjectDir(rootFolder())
+				.withTestKitDir(getTestKitDir())
 				.withPluginClasspath();
 	}
 
@@ -221,4 +222,12 @@ static String buildResultToString(BuildResult result) {
 			}
 		});
 	}
+
+	private static File getTestKitDir() {
+		String gradleUserHome = System.getenv("GRADLE_USER_HOME");
+		if (gradleUserHome == null || gradleUserHome.isEmpty()) {
+			gradleUserHome = new File(System.getProperty("user.home"), ".gradle").getAbsolutePath();
+		}
+		return new File(gradleUserHome, "testkit");
+	}
 }