Use basedir as forked JVM's working directory in Maven plugin

Fixes gh-16811

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/it/run-fork/src/main/java/org/test/SampleApplication.java

 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2019 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.
@@ -16,10 +16,12 @@
 
 package org.test;
 
+import java.io.File;
+
 public class SampleApplication {
 
 	public static void main(String[] args) {
-		System.out.println("I haz been run");
+		System.out.println("I haz been run from '" + new File("").getAbsolutePath() + "'");
 	}
 
 }

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/it/run-fork/verify.groovy

 def file = new File(basedir, "build.log")
-return file.text.contains("I haz been run")
+return file.text.contains("I haz been run from '$basedir'")
 

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/AbstractRunMojo.java

@@ -281,7 +281,10 @@ public abstract class AbstractRunMojo extends AbstractDependencyFilterMojo {
 		addClasspath(args);
 		args.add(startClassName);
 		addArgs(args);
-		runWithForkedJvm(this.workingDirectory, args, determineEnvironmentVariables());
+		runWithForkedJvm(
+				(this.workingDirectory != null) ? this.workingDirectory
+						: this.project.getBasedir(),
+				args, determineEnvironmentVariables());
 	}
 
 	/**