Fix Gradle plugin's tests following JUnit 5 upgrade

Closes gh-322

Author: wilkinsona

spring-javaformat-gradle/spring-javaformat-gradle-plugin/build.gradle

@@ -20,6 +20,7 @@ jar {
 }
 
 test {
+	useJUnitPlatform()
 	testLogging {
 		events "passed", "skipped", "failed"
 	}

spring-javaformat-gradle/spring-javaformat-gradle-plugin/src/test/java/io/spring/javaformat/gradle/CheckTaskTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017-2021 the original author or authors.
+ * Copyright 2017-2022 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.
@@ -45,11 +45,11 @@
 @ExtendWith(GradleBuildExtension.class)
 public class CheckTaskTests {
 
+	private final GradleBuild gradleBuild = new GradleBuild();
+
 	@TempDir
 	public File temp;
 
-	public GradleBuild gradleBuild;
-
 	@Test
 	public void checkOk() throws IOException {
 		BuildResult result = this.gradleBuild.source("src/test/resources/check-ok").build("check");

spring-javaformat-gradle/spring-javaformat-gradle-plugin/src/test/java/io/spring/javaformat/gradle/FormatTaskTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017-2021 the original author or authors.
+ * Copyright 2017-2022 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.
@@ -39,7 +39,7 @@
 @ExtendWith(GradleBuildExtension.class)
 public class FormatTaskTests {
 
-	public GradleBuild gradleBuild;
+	private final GradleBuild gradleBuild = new GradleBuild();
 
 	@Test
 	public void checkOk() throws IOException {