Expose AssertionErrors as is

Closes gh-9760

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

@@ -186,7 +186,7 @@ class AbstractContextLoader<T extends ConfigurableApplicationContext, L extends
 				T ctx = handler.load();
 				contextHandler.handle(ctx);
 			}
-			catch (RuntimeException ex) {
+			catch (RuntimeException | AssertionError ex) {
 				throw ex;
 			}
 			catch (Throwable ex) {

spring-boot-test/src/test/java/org/springframework/boot/test/context/StandardContextLoaderTests.java

@@ -194,6 +194,18 @@ public class StandardContextLoaderTests {
 				});
 	}
 
+	@Test
+	public void assertionErrorsAreAvailableAsIs() {
+		try {
+			this.contextLoader.load(context -> {
+				fail("This is expected");
+			});
+		}
+		catch (AssertionError ex) {
+			assertThat(ex.getMessage()).isEqualTo("This is expected");
+		}
+	}
+
 	@Configuration
 	static class ConfigA {