Allow JSON Testers to be @Autowired

Switch `@AutoConfigureJsonTesters` to use regular `@Autowired` injection
for JSON testers. Prior to this commit JSON Tester fields were
initialized directly which caused IDE issues and was also a little
confusing.

Fixes gh-6451

spring-boot-samples/spring-boot-sample-test/src/test/java/sample/test/service/VehicleDetailsJsonTests.java

@@ -19,6 +19,7 @@ package sample.test.service;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 
+import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.autoconfigure.json.JsonTest;
 import org.springframework.boot.test.json.JacksonTester;
 import org.springframework.test.context.junit4.SpringRunner;
@@ -34,6 +35,7 @@ import static org.assertj.core.api.Assertions.assertThat;
 @JsonTest
 public class VehicleDetailsJsonTests {
 
+	@Autowired
 	private JacksonTester<VehicleDetails> json;
 
 	@Test