Fix Bean Condition on GsonTester FactoryBean

This commit fixes the `@ConditionalOnClass` typo on the GsonTester
FactoryBean, which was testing for `ObjectMapper` instead of `Gson`.

This means that the next condition `@ConditionalOnBean(Gson.class)`
throws a `ClassNotFoundException: com.google.gson.Gson` since it's
trying to load that class to check for an existing Bean of that type.

Fixes gh-6502

spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/json/JsonTestersAutoConfiguration.java

@@ -77,7 +77,7 @@ public class JsonTestersAutoConfiguration {
 
 	@Bean
 	@Scope("prototype")
-	@ConditionalOnClass(ObjectMapper.class)
+	@ConditionalOnClass(Gson.class)
 	@ConditionalOnBean(Gson.class)
 	public FactoryBean<GsonTester<?>> gsonTesterFactoryBean(Gson gson) {
 		return new JsonTesterFactoryBean<GsonTester<?>, Gson>(GsonTester.class, gson);