Allow Matrix tests to select single artifact in a group

buildSrc/src/main/groovy/org/springframework/restdocs/build/matrix/MatrixTestExtension.groovy

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2018 the original author or authors.
+ * Copyright 2014-2019 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.
@@ -43,7 +43,7 @@ public class MatrixTestExtension {
 	void configureTestTasks(Project project) {
 		if (!entries.empty) {
 			cartesianProduct(entries.collect { entry ->
-				entry.versions.collect { ['group': entry.group, 'version': it] }
+				entry.versions.collect { ['group': entry.group, 'artifact': entry.artifact, 'version': it] }
 			}).forEach { configureTestTask(project, it) }
 		}
 	}
@@ -64,6 +64,7 @@ public class MatrixTestExtension {
 				resolutionStrategy.eachDependency { dependency ->
 					versionSelectors
 							.findAll{ it.group == dependency.requested.group }
+							.findAll { !it.artifact || it.artifact == dependency.requested.name }
 							.each { dependency.useVersion it.version }
 				}
 			}
@@ -98,6 +99,8 @@ public class MatrixTestExtension {
 
 		String group
 
+		String artifact
+
 		List<String> versions
 
 	}