Add Configurations class

Add a general purpose `Configurations` class that encapsulates the
sorting and merging rules that are usually apply. The class is
particularly useful in tests where configuration classes often need
to be specified, but an `@Import` or `ImportSelector` cannot be easily
used.

Two `Configurations` subclasses have been initially added. The
`UserConfigurations` class can be used to represent user defined
configuration and the `AutoConfigurations` class can be used to
represent a subset of auto-configurations. Auto configurations are
sorted using the same `@AutoConfiguraionBefore`/`@AutoConfiguraionAfter`
logic as the `@EnableAutoConfiguration` annotation.

Fixes gh-9795

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/AutoConfigurationsTests.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2012-2017 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.autoconfigure;
+
+import org.junit.Test;
+
+import org.springframework.boot.context.annotation.Configurations;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link AutoConfigurations}.
+ *
+ * @author Phillip Webb
+ */
+public class AutoConfigurationsTests {
+
+	@Test
+	public void ofShouldCreateOrderedConfigurations() throws Exception {
+		Configurations configurations = AutoConfigurations.of(AutoConfigureA.class,
+				AutoConfigureB.class);
+		assertThat(Configurations.getClasses(configurations))
+				.containsExactly(AutoConfigureB.class, AutoConfigureA.class);
+	}
+
+	@AutoConfigureAfter(AutoConfigureB.class)
+	public static class AutoConfigureA {
+
+	}
+
+	public static class AutoConfigureB {
+
+	}
+
+}