Prior to this commit, the Spring TestContext Framework only supported
loading an ApplicationContext in integration tests from either XML or
Java Properties files (since Spring 2.5), and Spring 3.1 introduced
support for loading an ApplicationContext in integration tests from
annotated classes (e.g., @Configuration classes). All of the
ContextLoader implementations used to provide this support load a
GenericApplicationContext. However, a GenericApplicationContext is not
suitable for testing a web application since a web application relies on
an implementation of WebApplicationContext (WAC).

This commit makes it possible to integration test Spring-powered web
applications by adding the following functionality to the Spring
TestContext Framework.

 - Introduced AbstractGenericWebContextLoader and two concrete
   subclasses:
   - XmlWebContextLoader
   - AnnotationConfigWebContextLoader

 - Pulled up prepareContext(context, mergedConfig) from
   AbstractGenericContextLoader into AbstractContextLoader to allow it
   to be shared across web and non-web context loaders.

 - Introduced AnnotationConfigContextLoaderUtils and refactored
   AnnotationConfigContextLoader accordingly. These utils are also used
   by AnnotationConfigWebContextLoader.

 - Introduced a new @WebAppConfiguration annotation to denote that the
   ApplicationContext loaded for a test should be a WAC and to configure
   the base resource path for the root directory of a web application.

 - Introduced WebMergedContextConfiguration which extends
   MergedContextConfiguration with support for a baseResourcePath for
   the root directory of a web application.

 - ContextLoaderUtils.buildMergedContextConfiguration() now builds a
   WebMergedContextConfiguration instead of a standard
   MergedContextConfiguration if @WebAppConfiguration is present on the
   test class.

 - Introduced a configureWebResources() method in
   AbstractGenericWebContextLoader that is responsible for creating a
   MockServletContext with a proper ResourceLoader for the
   resourceBasePath configured in the WebMergedContextConfiguration. The
   resulting mock ServletContext is set in the WAC, and the WAC is
   stored as the Root WAC in the ServletContext.

 - Introduced a WebTestExecutionListener that sets up default thread
   local state via RequestContextHolder before each test method by using
   the MockServletContext already present in the WAC and by creating a
   MockHttpServletRequest, MockHttpServletResponse, and
   ServletWebRequest that is set in the RequestContextHolder. WTEL also
   ensures that the MockHttpServletResponse and ServletWebRequest can be
   injected into the test instance (e.g., via @Autowired) and cleans up
   thread locals after each test method.

 - WebTestExecutionListener is configured as a default
   TestExecutionListener before DependencyInjectionTestExecutionListener

 - Extracted AbstractDelegatingSmartContextLoader from
   DelegatingSmartContextLoader and introduced a new
   WebDelegatingSmartContextLoader.

 - ContextLoaderUtils now selects the default delegating ContextLoader
   class name based on the presence of @WebAppConfiguration on the test
   class.

 - Tests in the spring-test-mvc module no longer use a custom
   ContextLoader to load a WebApplicationContext. Instead, they now
   rely on new core functionality provided in this commit.

Issue: SPR-5243