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/test/java/org/springframework/test/web/reactive/server/samples/SoftAssertionTests.java

@@ -0,0 +1,75 @@
+/*
+ * Copyright 2002-2021 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 org.springframework.test.web.reactive.server.samples;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.test.web.reactive.server.WebTestClient;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+
+/**
+ * Samples of tests using {@link WebTestClient} with soft assertions.
+ *
+ * @author Michał Rowicki
+ * @since 5.3
+ */
+public class SoftAssertionTests {
+
+	private WebTestClient client;
+
+
+	@BeforeEach
+	public void setUp() throws Exception {
+		this.client = WebTestClient.bindToController(new TestController()).build();
+	}
+
+
+	@Test
+	public void test() throws Exception {
+		this.client.get().uri("/test")
+				.exchange()
+				.expectAllSoftly(
+						exchange -> exchange.expectStatus().isOk(),
+						exchange -> exchange.expectBody(String.class).isEqualTo("It works!")
+				);
+	}
+
+	@Test
+	public void testAllFails() throws Exception {
+		assertThatExceptionOfType(AssertionError.class).isThrownBy(() ->
+				this.client.get().uri("/test")
+						.exchange()
+						.expectAllSoftly(
+								exchange -> exchange.expectStatus().isBadRequest(),
+								exchange -> exchange.expectBody(String.class).isEqualTo("It won't work :(")
+						)
+		).withMessage("[0] Status expected:<400 BAD_REQUEST> but was:<200 OK>\n[1] Response body expected:<It won't work :(> but was:<It works!>");
+	}
+
+
+	@RestController
+	static class TestController {
+
+		@GetMapping("/test")
+		public String handle() {
+			return "It works!";
+		}
+	}
+}