Update BootRun to support Gradle's configuration cache

See gh-22922

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/com/example/main/CustomMainClass.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2012-2020 the original author or authors.
+ *
+ * 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
+ *
+ *      https://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.example.main;
+
+/**
+ * Application used for testing {@code BootRun}'s main class configuration.
+ *
+ * @author Andy Wilkinson
+ */
+public class CustomMainClass {
+
+	protected CustomMainClass() {
+
+	}
+
+	public static void main(String[] args) {
+		System.out.println(CustomMainClass.class.getName());
+	}
+
+}