Support classes AND locations in @ContextConfiguration

Prior to this commit, the Spring TestContext Framework did not support
the declaration of both 'locations' and 'classes' within
@ContextConfiguration at the same time.

This commit addresses this in the following manner:

 - ContextConfigurationAttributes no longer throws an
   IllegalArgumentException if both 'locations' and 'classes' are
   supplied to its constructor.

 - Concrete SmartContextLoader implementations now validate the
   supplied MergedContextConfiguration before attempting to load the
   ApplicationContext. See validateMergedContextConfiguration().

 - Introduced tests for hybrid context loaders like the one used in
   Spring Boot. See HybridContextLoaderTests.

 - Updated the Testing chapter of the reference manual so that it no
   longer states that locations and classes cannot be used
   simultaneously, mentioning Spring Boot as well.

 - The Javadoc for @ContextConfiguration has been updated accordingly.

 - Added hasLocations(), hasClasses(), and hasResources() convenience
   methods to MergedContextConfiguration.

Issue: SPR-11634