Support ApplicationContextInitializers in the TCF

[sbrannen]

• Added initializers and inheritInitializers attributes to
  @ContextConfiguration.
  • Introduced support for ApplicationContextInitializers in
    ContextConfigurationAttributes, MergedContextConfiguration, and
    ContextLoaderUtils.
  • Introduced resolveInitializerClasses() in ContextLoaderUtils.
  • MergedContextConfiguration stores context initializer classes as a
    Set and incorporates them into the implementations of hashCode() and
    equals().
  • ApplicationContextInitializers are invoked in the new
    prepareContext(GenericApplicationContext, MergedContextConfiguration)
    method in AbstractGenericContextLoader.
  • Updated DelegatingSmartContextLoader to support initializers.
    Specifically, a test class may optionally declare neither XML
    configuration files nor annotated classes and instead declare only
    application context initializers. In such cases, an attempt will
    still be made to detect defaults, but their absence will not result
    an an exception.
  • Documented support for application context initializers in the
    testing chapter of the reference manual.

Issue: SPR-9011