Support automatic discovery of default TELs

Prior to this commit, there was no declarative mechanism for a custom
TestExecutionListener to be registered as a default
TestExecutionListener.

This commit introduces support for discovering default
TestExecutionListener implementations via the SpringFactoriesLoader
mechanism. Specifically, the spring-test module declares all core
default TestExecutionListeners under the
org.springframework.test.context.TestExecutionListener key in its
META-INF/spring.factories properties file, and third-party frameworks
and developers can contribute to the list of default
TestExecutionListeners in the same manner.

 - AbstractTestContextBootstrapper uses the SpringFactoriesLoader to
   look up the class names of all registered default
   TestExecutionListeners and sorts the instantiated listeners using
   AnnotationAwareOrderComparator.

 - DefaultTestContextBootstrapper and WebTestContextBootstrapper now
   rely on the SpringFactoriesLoader mechanism for finding default
   TestExecutionListeners instead of hard coding fully qualified class
   names.

 - To ensure that default TestExecutionListeners are registered in the
   correct order, each can implement Ordered or declare @Order.

 - AbstractTestExecutionListener and all default TestExecutionListeners
   provided by Spring now implement Ordered with appropriate values.

 - Introduced "copy constructors" in MergedContextConfiguration and
   WebMergedContextConfiguration

 - SpringFactoriesLoader now uses AnnotationAwareOrderComparator
   instead of OrderComparator.

Issue: SPR-11466
Merge from sbrannen/SPR-11466

* SPR-11466:
  Support automatic discovery of default TELs