Introduce soft assertions for WebTestClient

It happens very often that WebTestClient is used in heavyweight
integration tests, and it's a hindrance to developer productivity to
fix one failed assertion after another. Soft assertions help a lot by
checking all conditions at once even if one of them fails.

This commit introduces a new expectAllSoftly(..) method in
WebTestClient to address this issue.

client.get().uri("/hello")
	.exchange()
	.expectAllSoftly(
		spec -> spec.expectStatus().isOk(),
		spec -> spec.expectBody(String.class).isEqualTo("Hello, World")
	);

Closes gh-26969

spring-test/src/main/java/org/springframework/test/web/reactive/server/DefaultWebTestClient.java

@@ -21,6 +21,7 @@ import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
 import java.time.Duration;
 import java.time.ZonedDateTime;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.LinkedHashMap;
 import java.util.List;
@@ -507,6 +508,24 @@ class DefaultWebTestClient implements WebTestClient {
 			Flux<T> body = this.response.bodyToFlux(elementTypeRef);
 			return new FluxExchangeResult<>(this.exchangeResult, body);
 		}
+
+		@Override
+		public ResponseSpec expectAllSoftly(ResponseSpecMatcher... asserts) {
+			List<String> failedMessages = new ArrayList<>();
+			for (int i = 0; i < asserts.length; i++) {
+				ResponseSpecMatcher anAssert = asserts[i];
+				try {
+					anAssert.accept(this);
+				}
+				catch (AssertionError assertionException) {
+					failedMessages.add("[" + i + "] " + assertionException.getMessage());
+				}
+			}
+			if (!failedMessages.isEmpty()) {
+				throw new AssertionError(String.join("\n", failedMessages));
+			}
+			return this;
+		}
 	}
 
 

spring-test/src/main/java/org/springframework/test/web/reactive/server/WebTestClient.java

@@ -845,6 +845,11 @@ public interface WebTestClient {
 		 * about a target type with generics.
 		 */
 		<T> FluxExchangeResult<T> returnResult(ParameterizedTypeReference<T> elementTypeRef);
+
+		/**
+		 * Array of assertions to test together a.k.a. soft assertions.
+		 */
+		ResponseSpec expectAllSoftly(ResponseSpecMatcher... asserts);
 	}
 
 
@@ -1006,4 +1011,5 @@ public interface WebTestClient {
 		EntityExchangeResult<byte[]> returnResult();
 	}
 
+	interface ResponseSpecMatcher extends Consumer<ResponseSpec> {}
 }