Add support to ContextLoader for configuring context's parent

Closes gh-9749

spring-boot-test/src/main/java/org/springframework/boot/test/context/AbstractContextLoader.java

@@ -62,6 +62,8 @@ class AbstractContextLoader<T extends ConfigurableApplicationContext, L extends
 
 	private ClassLoader classLoader;
 
+	private ApplicationContext parent;
+
 	protected AbstractContextLoader(Supplier<T> contextSupplier) {
 		this.contextSupplier = contextSupplier;
 	}
@@ -115,6 +117,12 @@ class AbstractContextLoader<T extends ConfigurableApplicationContext, L extends
 		return self();
 	}
 
+	@Override
+	public L parent(ApplicationContext parent) {
+		this.parent = parent;
+		return self();
+	}
+
 	/**
 	 * Add the specified auto-configuration classes.
 	 * @param autoConfigurations the auto-configuration classes to add
@@ -226,6 +234,9 @@ class AbstractContextLoader<T extends ConfigurableApplicationContext, L extends
 
 	private T configureApplicationContext() {
 		T context = AbstractContextLoader.this.contextSupplier.get();
+		if (this.parent != null) {
+			context.setParent(this.parent);
+		}
 		if (this.classLoader != null) {
 			Assert.isInstanceOf(DefaultResourceLoader.class, context);
 			((DefaultResourceLoader) context).setClassLoader(this.classLoader);

spring-boot-test/src/main/java/org/springframework/boot/test/context/ContextLoader.java

@@ -134,6 +134,16 @@ public interface ContextLoader {
 	 */
 	ContextLoader classLoader(ClassLoader classLoader);
 
+	/**
+	 * Configure the
+	 * {@link org.springframework.context.ConfigurableApplicationContext#setParent(ApplicationContext)
+	 * parent} of the {@link ApplicationContext}.
+	 *
+	 * @param parent the parent
+	 * @return this instance
+	 */
+	ContextLoader parent(ApplicationContext parent);
+
 	/**
 	 * Create and refresh a new {@link ApplicationContext} based on the current state of
 	 * this loader. The context is consumed by the specified {@code consumer} and closed