Make AbstractApplicationContextRunner public

Fixes gh-11929

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/AbstractApplicationContextRunner.java

@@ -95,7 +95,7 @@ import org.springframework.util.Assert;
  * @see ReactiveWebApplicationContextRunner
  * @see ApplicationContextAssert
  */
-abstract class AbstractApplicationContextRunner<SELF extends AbstractApplicationContextRunner<SELF, C, A>, C extends ConfigurableApplicationContext, A extends ApplicationContextAssertProvider<C>> {
+public abstract class AbstractApplicationContextRunner<SELF extends AbstractApplicationContextRunner<SELF, C, A>, C extends ConfigurableApplicationContext, A extends ApplicationContextAssertProvider<C>> {
 
 	private final Supplier<C> contextFactory;