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.

- 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 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 CacheAwareContextLoader 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 CacheAwareContextLoader for loading
  contexts.
- Introduced getParentApplicationContext() in MergedContextConfiguration
- the loadContext(MergedContextConfiguration) methods in
  AbstractGenericContextLoader and AbstractGenericWebContextLoader now
  set the parent context as appropriate.

Issue: SPR-5613, SPR-9863