Auto-configure codecs in WebTestClient

This commit applies what's been done in gh-9166 for WebFlux client and
server, but for the `WebTestClient` auto-configuration.

`WebTestClient` can be configured for mock or integration tests and this
change applies `CodecCustomizer` beans to the client being built.

Closes gh-9577

spring-boot-test/pom.xml

@@ -105,6 +105,11 @@
 			<artifactId>spring-web</artifactId>
 			<optional>true</optional>
 		</dependency>
+		<dependency>
+			<groupId>org.springframework</groupId>
+			<artifactId>spring-webflux</artifactId>
+			<optional>true</optional>
+		</dependency>
 		<dependency>
 			<groupId>net.sourceforge.htmlunit</groupId>
 			<artifactId>htmlunit</artifactId>
@@ -162,11 +167,6 @@
 			<artifactId>spring-webmvc</artifactId>
 			<scope>test</scope>
 		</dependency>
-		<dependency>
-			<groupId>org.springframework</groupId>
-			<artifactId>spring-webflux</artifactId>
-			<scope>test</scope>
-		</dependency>
 	</dependencies>
 	<build>
 		<plugins>