Provide support for context hierarchies in the TCF

Prior to this commit the Spring TestContext Framework supported creating
only flat, non-hierarchical contexts. There was no easy way to create
contexts with parent-child relationships.

This commit addresses this issue by introducing a new @ContextHierarchy
annotation that can be used in conjunction with @ContextConfiguration
for declaring hierarchies of application contexts, either within a
single test class or within a test class hierarchy. In addition,
@DirtiesContext now supports a new 'hierarchyMode' attribute for
controlling context cache clearing for context hierarchies.

- Introduced a new @ContextHierarchy annotation.
- Introduced 'name' attribute in @ContextConfiguration.
- Introduced 'name' property in ContextConfigurationAttributes.
- TestContext is now aware of @ContextHierarchy in addition to
  @ContextConfiguration.
- Introduced findAnnotationDeclaringClassForTypes() in AnnotationUtils.
- Introduced resolveContextHierarchyAttributes() in ContextLoaderUtils.
- Introduced buildContextHierarchyMap() in ContextLoaderUtils.
- @ContextConfiguration and @ContextHierarchy may not be used as
  top-level, class-level annotations simultaneously.
- Introduced reference to the parent configuration in
  MergedContextConfiguration and WebMergedContextConfiguration.
- Introduced overloaded buildMergedContextConfiguration() methods in
  ContextLoaderUtils in order to handle context hierarchies separately
  from conventional, non-hierarchical contexts.
- Introduced hashCode() and equals() in ContextConfigurationAttributes.
- ContextLoaderUtils ensures uniqueness of @ContextConfiguration
  elements within a single @ContextHierarchy declaration.
- Introduced CacheAwareContextLoaderDelegate that can be used for
  loading contexts with transparent support for interacting with the
  context cache -- for example, for retrieving the parent application
  context in a context hierarchy.
- TestContext now delegates to CacheAwareContextLoaderDelegate for
  loading contexts.
- Introduced getParentApplicationContext() in MergedContextConfiguration
- The loadContext(MergedContextConfiguration) methods in
  AbstractGenericContextLoader and AbstractGenericWebContextLoader now
  set the parent context as appropriate.
- Introduced 'hierarchyMode' attribute in @DirtiesContext with a
  corresponding HierarchyMode enum that defines EXHAUSTIVE and
  CURRENT_LEVEL cache removal modes.
- ContextCache now internally tracks the relationships between contexts
  that make up a context hierarchy. Furthermore, when a context is
  removed, if it is part of a context hierarchy all corresponding
  contexts will be removed from the cache according to the supplied
  HierarchyMode.
- AbstractGenericWebContextLoader will set a loaded context as the
  ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE in the MockServletContext when
  context hierarchies are used if the context has no parent or if the
  context has a parent that is not a WAC.
- Where appropriate, updated Javadoc to refer to the
  ServletTestExecutionListener, which was introduced in 3.2.0.
- Updated Javadoc to avoid and/or suppress warnings in spring-test.
- Suppressed remaining warnings in code in spring-test.

Issue: SPR-5613, SPR-9863