Introduce MockEnvironment in the spring-test module

For legacy reasons, a MockEnvironment implementation already exists in multiple places within Spring's test suite; however, it is not available to the general public.

This commit promotes MockEnvironment to a first-class citizen in the spring-test module, alongside the existing MockPropertySource.

In addition, the following house cleaning has been performed.

 - deleted MockPropertySource from the spring-expression module
 - deleted MockEnvironment from the "spring" integration testing module
 - updated test copies of MockPropertySource and MockEnvironment
 - documented MockEnvironment and MockPropertySource in the testing
   chapter of the reference manual

Issue: SPR-9492

src/test/java/org/springframework/mock/env/MockEnvironment.java

@@ -1,60 +0,0 @@
-/*
- * Copyright 2002-2011 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.mock.env;
-
-import org.springframework.core.env.AbstractEnvironment;
-import org.springframework.core.env.ConfigurableEnvironment;
-
-/**
- * Simple {@link ConfigurableEnvironment} implementation exposing a
- * {@link #setProperty(String, String)} and {@link #withProperty(String, String)}
- * methods for testing purposes.
- * 
- * @author Chris Beams
- * @since 3.1
- * @see MockPropertySource
- */
-public class MockEnvironment extends AbstractEnvironment {
-
-	private MockPropertySource propertySource = new MockPropertySource();
-
-	/**
-	 * Create a new {@code MockEnvironment} with a single {@link MockPropertySource}.
-	 */
-	public MockEnvironment() {
-		getPropertySources().addLast(propertySource);
-	}
-
-	/**
-	 * Set a property on the underlying {@link MockPropertySource} for this environment.
-	 */
-	public void setProperty(String key, String value) {
-		propertySource.setProperty(key, value);
-	}
-
-	/**
-	 * Convenient synonym for {@link #setProperty} that returns the current instance.
-	 * Useful for method chaining and fluent-style use.
-	 * @return this {@link MockEnvironment} instance
-	 * @see MockPropertySource#withProperty(String, String)
-	 */
-	public MockEnvironment withProperty(String key, String value) {
-		this.setProperty(key, value);
-		return this;
-	}
-
-}