Migrate Hamcrest assertions to AssertJ

Migrate all existing `assertThat(..., Matcher)` assertions to AssertJ
and add checkstyle rules to ensure they don't return.

See gh-23022

spring-webmvc/spring-webmvc.gradle

@@ -65,6 +65,7 @@ dependencies {
 		exclude group: "xom", module: "xom"
 		exclude group: "xerces", module: "xercesImpl"
 	}
+	testCompile("org.xmlunit:xmlunit-assertj:2.6.2")
 	testCompile("org.xmlunit:xmlunit-matchers:2.6.2")
 	testCompile("org.hibernate:hibernate-validator:6.0.16.Final")
 	testCompile("io.projectreactor:reactor-core")